au where Lily is Sirius's younger sister

It was late on a Friday, and the Common Room was practically empty. The fire blazed in the hearth. To say James Potter was jittery would be an understatement. He was fucking terrified. Forget that Sirius was his best mate, because this was a new level. Of course he'd always tried to bury the feeling under denials, dates and snogging other girls. But something changed at the start of his Seventh Year.

Sirius was the first Black to get sorted into Gryffindor, but not the last. His younger sister, Lily, was sorted the following year and they'd all expected Regulus to follow suit the year after that. He hadn't. Either way, the Potters had pseudo-adopted Sirius and Lily. And now…

Well now James was pretty sure he was in love with her.

Sirius was going to kill him, when he found out. Absolutely slaughter him. James would have nowhere to hide. Remus and Peter would throw him back to Sirius in a second, so no help there. Marlene would just laugh for hours. Lily could never know because she only saw him like a brother, he was sure of it.

Red hair dropped over his shoulder. "Thinking hard, Potter?" Lily whispered lips just next to his ear. He felt his heart skip a few beats.

"Why would I be doing a thing like that?" He didn't dare turn his head. He knew if he did, all that would meet him was the shocking urge to kiss her. She laughed, and he felt her breath blow through his hair as she did. Merlin help him.

"Good point." She patted his shoulder, walking around the couch and flopping down next to him. She easily kicked her legs up, draping them across his lap. She had her Potions textbook with her, and she flipped it open to a dog-eared page. He just stared at her. It was stunning how genes worked, because although Sirius had dark hair and dark eyes, Lily had the recessive genes of her mother. Her red hair and bright green eyes made it a wonder she was a Black at all. At first people thought she'd been adopted or something, as if the Blacks could be that compassionate. Well, she could. Sirius sometimes. The rest…

James did enjoy looking at her almost as much as he enjoyed talking to her and just being near her. Her nose tipped up at the end, but it crinkled when she smiled or laughed. She hadn't laughed much around him when they were younger, but recently she'd crack up at even the lamest of jokes.

"You're staring." Lily had her nose crinkled now, a small smile donning her bright red face.

"You're blushing." James said stupidly.

"Because you're staring."

"Would you rather I'd be doing something else?"

"I'd rather you be doing a lot of other things." She flushed, looking down at her book again. James nearly reached ten before he breathed again. What had that meant? Did she want him to leave? Or did she want him to…

Only one was to find out.

James reached out, catching a finger under her jaw, and lifted her face up. She chewed her lip. He'd noticed because that's where he found himself staring. Closer. They needed to be closer. He slid his hand along the side of her face, until it cradled her jaw. Leaning forward, he could tell she was holding her breath. He was too. Finally, their lips touched. A small sigh escaped her mouth, and he pulled away. Eye contact. Lips crashing back together. This time they knew what they were doing.

It was a dream it had to be, but then she'd sigh again and good Godric it wasn't. This was happening! There was no way she didn't fancy him with this happening!

Suddenly James was yanked away by the collar of his shirt. "JUST WHAT THE BLOODY SODDING HELL DO YOU THINK YOU'RE DOING TO MY SISTER?"

Bugger.

"Sirius, calm down. It was just a snog." Lily crossed her arms defiantly.

"Calm down?" Sirius turned on her. "I come down here to see if my mate wants to visit Moony in the Hospital Wing, and he's STUCK ON YOUR FACE."

"DON'T TREAT ME LIKE I'M A CHILD!" Lily jumped to her feet, shouting up at her brother.

"YOU ARE A CHILD!"

"I'M SIXTEEN."

"EXACTLY!"

"YOU'RE IMPOSSIBLE." She shoved past him.

James didn't know if it was just to piss Sirius off more or because she really wanted to, but before she got to the stairs, Lily pressed a kiss against his lips. He very nearly pulled her back to him and did inappropriate things. He watched her as she climbed the staircase, until he couldn't anymore. James didn't know it, of course, but his face portrayed all the emotion he'd held down for years. Slowly but surely, he turned round to look at Sirius.

Absolutely slaughter him.

"That's my sister, James." His voice was dead calm.

"I know." James clenched his jaw, and looked Sirius in the eye.

"I don't know what you think you're doing," Sirius said. "But she's not just a girl you can shag and chuck alright?"

"I wasn't planning on it."

He scoffed. "You forget who you're talking to."

Silence.

"I love her."

"No you don't." Sirius sounded terrified.

"I do."

"No," Sirius grabbed him by the front of the shirt, slamming him against the wall. "You. Don't."

"Yes, I do." James shoved him away, and he was suddenly full of fury. "And I'll snog her if I want. And I'll date her if I want. I'll bloody well marry her if I want." He could be with anyone, it was true, but Lily was all he wanted. She was funny and clever and perfect. She filled in every gap in him, where he didn't know when to stop talking, or how to comfort someone. She was his best friend's little sister and he was in love with her.

"If you marry my sister—"

"Then we'll really be brothers." James finished. Sirius scowled, stomping back up to the dorms, probably to complain to Peter.

"James?" Came a soft voice from the bottom of the girls' stairs. Lily.

Bugger shit.

He looked over, as way of response. He didn't know how much she'd heard, or if she was scared away. Her expression was unreadable from the distance, and it wasn't likely that James was about to stand up and go see. She smiled, though, he could see that.

"I love you, too."