Prompt I got

"Why not some taboo Hiccstrid? Guess a modern AU would be the two of them being raised as siblings from adoptive parents. It may not "be" wrong, but to them, it feels wrong."

I'll confess, this thought had occurred when I was writing 'Adopted' and first made Hiccup and Astrid adopted siblings, but I didn't think much of it. Now someone has asked for it, the thought is back!

I think I'm gonna write all of this as a short (hopefully, we all know how that goes with me!) story, and post it all at once because I'm already juggling multiple stories but if I'm struggling to get past writers block, I work on one shots.

NOTE - the two are NOT biologically related. Just in case you didn't get that.

And I have babbled a huge authors note here. On with the adopted Hiccstrid!

-HTTYD-

Hiccup bit his lip, eyes straying no matter how he tried to fight it.

Astrid was doing it on purpose. She must be.

Of course, he could be wrong - its not like Astrid would have expected that wearing shorts would have her brother hot under the collar. Gods, those shorts. They rode higher since she gained a few inches in height, revealing miles of muscled thighs.

Summer was still in effect, so Astrid's usually milky skin was lightly tanned. The golden colour was illuminated by the soft white vest she had on, lazing in the back garden to sun herself. The sun lounger angle meant everything about her was practically glowing, from her golden-blonde hair currently resting in a braid on her shoulder to the exposed skin he really shouldn't be looking at so intently.

"That you Hiccup?"

Astrid twisted to where he was shamefully watching her from the shadows, lifting her sunglasses to fix him with those big blue eyes.

"What you doing over there?"

Since 'ogling my adopted sister which happens to be you' was not an appropriate answer, Hiccup shrugged.

"I can't stand the sun like you but wanted some air."

"Tough job?"

"Long job. Don't get me wrong, being paid to draw dragons is awesome but they want a dozen different species. I just needed a breather."

Astrid nodded, still twisted in her seat and offering him a terribly tempting view of her breasts pushed together by her arms. His stomach fizzled; he felt sick as much as he did pleased.

"Come over here."

Hiccups legs carried him before his brain could argue, sun beating down on skin already warmed by inappropriate arousal. Astrid patted the lounger space by her legs, sweet pink lips puckered slightly in a pout.

"What do you want?"

"Take off your shirt."

Hiccup almost choked. Crossing his arms across his skinny chest self-consciously.

"W-why?"

"Because you need sunblock. You burned through your clothes before, remember?"

"That's why I was in the shade."

"Well, now you're out in the sun. Come on, I've seen you topless before."

Swallowing thickly, Hiccup wasn't sure whether to call it a dream or a nightmare as he turned around, slipping his light linen shirt off. Astrid knelt behind him, heat pouring off her skin and soaking into his as she reached for the sunscreen. Her fingers rubbed the lotion into his back, short nails skimming his shoulders and he was so glad nobody could see his face. They might see him biting his lip, suppressing all sorts of pathetic, desperate sounds.

"Why so tense?"

Astrid's voice made him jump, jolting him from his reverie of ignoring the erection pulsing in his shorts.

"It's just weird. Wouldn't you feel strange if I was doing this?"

Her hands stilled for a second, barely enough to notice but definitely there.

"No. I mean... you're my brother. Why would it be weird?"

Hiccup closed his eyes, trying to focus on that word. Brother. His feelings were an absolute disgrace, and he needed to start remembering that.

"I dunno. Just is."

Astrid continued smoothing sunscreen over him, eventually stopping and Hiccup wasn't sure if he was relieved or disappointed. He hastened to put his shirt on, realising too late that the puddled fabric in his lap was hiding his... problem.

"I'm uh, just gonna go get some water."

"Bring me a drink from the fridge? Pretty please?"

Gods, Astrid was tempting like that, her pleading tone and soft pink lips. He kept his body angled specifically to ensure she couldn't see he was hard.

"Sure. What do you want?"

"Surprise me."

He all but ran into the house, getting a judgemental look from the elderly wolfhound on his dog bed. Almost like Toothless knew what he was thinking. Stormfly, Astrid's tabby cat, was snoozing on the side next to Cloudjumper - their mothers ginger cat. They were enjoying a patch of sunlight quite calmly, at odds with Hiccup and his trembling muscles.

Astrid... his sister made his blood boil for all the wrong reasons.

He grabbed a water bottle, then a fruit juice for Astrid. He placed his water bottle against his groin, hissing as the cold successfully killed his erection. Feeling a jot more decent to go back outside, Hiccup handed Astrid her drink and wavered between whether or not to sit on the other lounger. Astrid's obscene sound as cold juice filled her mouth put paid to the idea he could be near her, scuttling back into the shade.

Hiccup knew his feelings were awful. Depraved. He should just off himself and be done with it. He's known that since the day the twisted longing made itself known.

Three years ago. She was seventeen. He was sixteen. Astrid had always been just his big sister - she was adopted right before his mom found out she was pregnant with Hiccup, after they thought she couldn't have babies - she definitely couldn't have more, Hiccup's birth had been traumatic and difficult. She'd had to have a hysterectomy. Still, their parents were happy to have them both.

Then he had caught Astrid coming out of the shower, skin glistening with water droplets as she towelled her hair. She hadn't realised he saw her, all of her from long pale neck to her cute, small but utterly perfect breasts. The dip of her waist over the swell of her hips, the blonde curls that sat at the apex of her muscled thighs, all covered in creamy skin he ached to touch.

Astrid still didn't know about that. Hiccup had managed to hide for just long enough that she made it to her room, none-the-wiser that her brother had seen her. At first he thought the feeling was embarrassment. It was awkward to see your sister naked, right?

Then he started to think about it more. Started noticing the way her lips quirked up in a pout that he craved to feel against his own. He realised his hands would shake when Astrid asked him to braid her hair for her. The smell of her shampoo used to barely register with him; now Hiccup took in a deep breath full of the scent whenever he could.

Hiccup came to a horrifying conclusion.

He was attracted to his sister.

At first, he put it down to being a virgin. He needed to just see other bodies, ones that weren't Astrid. Needed some sexual experience.

That hadn't worked. Cami had been blonde and blue eyed, and despite her being softer all over than his sister, Hiccup could only see Astrid. Their relationship hadn't lasted long.

He needed something different.

Heather was different - dark hair, green eyes. Her mouth was always a deep shade of red, like she had just put on lipstick or been enjoying a cherry slush. He'd had higher hopes for Heather curing his sickness.

Embarrassingly, he could barely get it up and only biting her had stopped Astrid's name tumbling from his lips when he came.

Now Hiccup knew; he only wanted Astrid.

The one person he could never have.

Astrid's eyes sought him again, hiding back in the shade and she sighed.

"Now you're just avoiding me."

"Am not. It's too hot."

"Weirdo. Fine. Go back to your dragons, I'll just stay out here alone..."

Her voice took on that classic guilt-tripping tone, the one Astrid knew would make Hiccup do whatever she asked. Using the cold bottle to shrivel his erection down again, Hiccup sighed and stepped back out to sit awkwardly on the other lounger. She beamed like he had made her whole day, which only made him feel worse; he was her brother, and he was effectively leering at her like some perverted street creep.

Hiccup hated himself.

"You're the best. Here, rub that on yourself before you burn."

Hiccups hands shook as he rubbed the sunscreen onto his face and arms, then his skinny legs.

"Don't forget your chest since you burn through clothes, unless you're gonna lie on your front I guess."

He wasn't going to, but when Astrid dropped her phone and twisted in her seat, green lace briefs visible over the line of her shorts, he had little choice if he wanted to avoid her seeing he was hard again.

The problem was; he had to have his head facing one side or the other and Astrid would probably be offended if he didn't face her. So he was stuck with an erection trapped between stomach and chair, Astrid's bare skin barely impeded by her limited clothing in his vision. At least he could close his eyes frequently against the sun.

"Did you really demand I stay out here to sit in silence with you?"

"You're always so busy... I just miss you Hic."

That made him feel worse. He did avoid Astrid sometimes, needing a break from seeing the object of his twisted desires which was difficult when they lived in the same house. It wasn't that Hiccup couldn't afford to move out, their family had money and he did pretty well off his art jobs. He could get a job in a coffee shop or something to pay bills and what not.

But he just loved the family home. They had grown up in it, surrounded by their mother Valka's curious paintings and their father Stoick's car parts, learning to swim in the pool with haddocks painted along the tiles. Exploring the forests nearby for hours at a time. With his sister... his sister that was threatening to drive him out so he could wrestle with these accursed, disgusting desires alone.

"I know. I'm sorry Astrid."

He was apologising for so much more than just being 'too busy'. For a second, he feared Astrid could tell that much. Instead, they heard the front door rattle, the floor shake as their enormous father entered. Valka peered out the open back door, smiling when she saw the two of them out in the sunshine.

"Henry, out in the sun?"

"Blame Astrid. It's too hot!"

He complained, winning amused laughter from their mom as she observed him sulking on a sun lounger.

"Why not go for a swim to cool down son?"

"That's a great idea! I'm gonna go change."

Astrid leapt up, leaving Hiccup alone cursing the suggestion of his mother. Now he was going to have to put on swim trunks and try to hide the fact Astrid in her fucking bikini was basically a wet dream waiting to happen.

"You getting up Henry?"

"In a minute."

He had to wait for his mother to not be looking; he was still half-hard even though Astrid had vanished from sight. Heading up to his room at the speed of light before either parent could notice how flustered he was, Hiccup pushed down his shorts and grasped his cock. A glance through the window offered him the sight of Astrid already out by the pool, even more sinfully bare skin on show now.

His cock pulsed in his hand as that image seared itself into his brain, Astrid's body the only fuel he needed to stroke himself until he spilled over his hand. Wiping himself down, Hiccup hoped that would at least slow down his bodies reaction to Astrid. Pulling on the sunproof rashguard vest he needed for swimming and his trunks, he headed back down to the torturous heaven that was Astrid splashing around all of two feet away from him in approximately nothing but a little fabric over her breasts and groin.

"Come on in Hiccup, the water is great!"

"I wish you wouldn't call him that Astrid, it's not very nice to your brother."

"He doesn't mind! He introduces himself as Hiccup! Even dad calls him Hiccup."

Valka rolled her eyes, watching fondly as Hiccup slid into the pool water and bobbed awkwardly. Astrid was already wet through, bikini clinging criminally tight to her skin and hair damp down her back. It wasn't fair.

"Come on bro, you're meant to swim in a swimming pool!"

"I know, I'm waiting to see if you intend to stay in one place so I don't swim into you."

Because you might notice how much I like your body close to mine.

Astrid fucking winked at him, tongue peeking out cutely between her lips and Hiccup panicked as his shorts tightened again. His daily life involved avoiding these situations, and here was Astrid hitting every single button.

"Don't worry, I promise not to pull your shorts down like when we were kids."

Gods, she needed to stop.

"I should think not."

Hiccup managed to do a few lengths, but he stayed firmly on his front. He feared traumatising their parents, who were in and out of the garden chatting between making dinner, if he did a backstroke. He also feared his mothers cooking, but that was an entirely different fear altogether.

"Ok, I'm done. I've put off work for long enough."

Astrid pouted sulkily, but he insisted and managed to get out of the pool before she attempted to trap him in there. There would be no hiding from her if she did that. He managed to avoid the notice of his parents by rushing past for a shower, changing into jeans that would at least sort of conceal his reaction to Astrid eating dinner - as if there wasn't enough torture, they were having spaghetti today. Watching Astrid slurp and suck the strands between her plush lips... it was heaven and hell in one.

He sat back at his desk and tried to focus back on work, successfully distracting himself for all of ten minutes before the blonde goddess was at his door, back in those shorts again and leaning against the door frame casually, looking enticing as fuck with those tempting legs, her beautiful eyes.

"Sup?"

"Mom sent me to get you for dinner."

"Why not just shout me?"

"Because I wanted to see you. You ran away too fast downstairs."

For good reason, Hiccup mused. Astrid didn't know that though.

"I have to work."

She sighed, smiling tightly.

"I know, but that doesn't mean I don't miss you. We used to be best friends, now even though we live together I hardly ever see you."

Fists clenching beneath his desk, he felt guilt surge.

"I'm sorry. I didn't know you felt that way."

"Mom and dad are going away for that animal thing next week... think you'll be done with this project by then?"

Hiccup's heart pounded.

"I don't know. Maybe. Why?"

She gave him a toothy smile, all innocence and mischief.

"Because it means we have a few days to ourselves, we can hang out."

"We can hang out when mom and dad are home you know."

"I know, but it'll be just us so you're obligated to keep me company then. Thems the rules."

Swallowing thickly as she turned and skipped off, Hiccup had conveniently forgotten they would be left alone. It didn't mean anything different to Astrid really, but for Hiccup it was two less people to make him feel guilty about his feelings. That was neither a good or bad thing, he mused. The guilt made him sick, but he was still sick for allowing the feelings to start with. So it was neither a win or a loss.

He realised he was supposed to be going down for dinner, splashing cold water across his face to try and cool his flushed skin before he took himself downstairs. Hiccup grit his teeth as he saw Astrid already slurping away, trying not to imagine how those lips would feel against his own. How they would look on various parts of his body.

Gods, he really had problems.

Hiccup focused on the garlic bread they had with their dinner. There was nothing sexual about garlic bread. He ate his spaghetti in bites rather than by strand, doing everything he could not to look at Astrid as she licked her lips to chase a little sauce. By the time they were done devouring the icecream cake for afters, Hiccup was so full he couldn't think about sex.

Well, so he thought.

Until he realised Astrid's usually flat stomach was a little round, peeking out as a tempting strip of skin under her vest. Despite the three empty chairs in the living room, Astrid slung herself across the sofa Hiccup was sat on, little fluffy slippers nudging his thigh as her legs constantly invaded his peripheral vision. Playing it off as uncomfortable, Hiccup pulled the cushion from behind him and rested it on his lap, hiding his unforgivable response to his sister sat next to him.

Despite how badly he needed the distraction, Hiccup had completed the large design job before their parents left. Astrid was ecstatic that he wouldn't be too busy for her, and Hiccup knew he was completely fucked.

-HTTYD-

I don't really know what to call this ship... Hiccstrid with a twist? Adoptstrid? Hmm. Does it matter?