The next night, Lincoln was so full of anticipation and lust that he could not prevent his forming erection leaking precum in the same undies he wore yesterday. He did not shower during the day either because he wanted Lisa to experience his natural scent without any pesky traces of soap. He wondered how long he could get away without showering until his sisters complained that he was too stinky to be around.

"That'll be my side experiment" he chuckled to himself.

As expected, his bedroom door slowly opened and he was happy and relieved to see his genius sister.

"Hello Lincoln" she smiled after she closed the door behind her.

"Hey Lisa, are you ready to continue our experiment?"

"Of course, I expect more than stellar results tonight"

"I hope I can rise to the occasion, hop in" he replied, before lifting his covers.

Lisa eagerly climbed into bed with her big brother and immediately his smell assaulted her olfactory senses.

"Shall I start with your malodorous feet again?" she winked.

"Soon…soon, why don't you just cuddle up with me for a while?"

"I don't see what this has to do with the experiment but I have no reason to decline"

The little genius went to nuzzle her cheek against Lincoln's but before she could he, somewhat forcefully cupped his hand in her shaggy hair and pressed her face into his armpit. Lisa began to struggle but the moment she inhaled through her button nose her limbs ceased to work.

"Do you like my stinky armpit Lisa?" he grinned almost malevolently.

There was no reply apart from the noise of his little sister sniffing loudly.

"My god Lincoln…" she uttered. "The scent emanating from your underarm rivals the scent from your feet"

"I'll take that as a compliment" he chuckled.

Lisa wrapped her arms around her brother's slender torso as she nuzzled her face further into his armpit. He sighed contentedly enjoying the closeness but when he felt Lisa's tongue gently press against his hairless underarm he could not resist slipping a hand in his undies and gripping his twitching member. Hearing his little sister moan erotically as she slid her wet tongue around his stinky pit made him quickly glide his hand up and down his length. His pace quickened even further when Lisa started humping his torso, lost in a frenzy of sexual intensity.

"Lincoln…" she whimpered. "Th…This pleasure I'm experiencing is…"

"Is what?" he replied through rapid breathing.

"A little frightening…"

Once again he felt a pang of guilt knowing she was not used to orgasms like he was. From the day he first learned how to masturbate, hardly a night went by where he left his dick alone. He wanted to tell her to power through because the end result would feel spectacular but his brotherly love overshadowed his lust momentarily to the point where he released the grip on his member and embraced her whilst she continued to lick him.

"Lisa, let's stop for a sec"

He pulled her closer so her face slid from his armpit to his chest. Her breathing was erratic so he caressed her unkempt hair until she calmed down. They stayed silent for a little while, both enjoying the closeness even though Lincoln's erection refused to yield.

"Do you know about orgasms Lisa?" he asked, breaking the silence.

"Well, I have the gist I guess. I believe it's the result of experiencing pleasure so intense that the body can not handle so it has to release the pressure through ejaculation. Is that correct?"

"Uhh…yeah that's right. Do you want me to show you on myself?"

"Hmm, I am intrigued, especially since I've been inhaling your odour"

His cock twitched again at that comment.

"Didn't you experience something similar last night when I…you know… pressed my big toe against your…"

"My vagina?"

"Well, yes…"

"I must admit I have never felt anything that pleasurable since I dreamt about meeting Pythagoras and Hippocrates "

He laughed quietly at her statement and hugged her even tighter.

"Alright, this won't take long…thanks to you" he winked.

Without another word he slipped his underwear down to his ankles and flicked them onto the floor with his foot. Lisa's eyes widened once she gazed at his hairless, erect cock. To her it was a glorious sight that made her virgin entrance moisten. When Lincoln gripped his member he slid his free hand from Lisa's hair and snaked it down her lithe frame until it cupped her butt causing her to sigh deeply. Not even a minute later after frantically masturbating, Lincoln could not hold back his grunts and moans. Lisa stared in awe at his cock spurting cum across his torso, a decent amount pooling in his navel. Whilst Lincoln was recovering from his intense orgasm, Lisa sat up and kneeled towards his chest and scooped up a glob her big brother's cum with her index and middle finger.

"So th…this is semen…" she uttered, barely able to contain herself.

Lisa rubbed her cum covered fingers against her thumb, intrigued by the consistency. She then put her finger's up to her nose, a thick string of cum still attached to her thumb. Sniffing the substance confused her olfactory senses, she could not tell if it was pleasant or not. Even though she was hesitant her lust defeated her inhibitions once more so she gingerly stuck the tip of her tongue out and licked. The taste was peculiar, her brain could not make a specific flavour but because a part of her still thought of these antics as an experiment she put her slimy fingers into her mouth so she could properly taste her brother's essence. Lisa found that the taste was delectable and she craved more. Once she had licked her fingers clean she bent down and greedily sucked the cum out of his belly button.

"I'm not scared anymore Lincoln. Please…help me achieve the same euphoria I experienced last night"

The snowy haired boy smiled at her and nodded.

"Climb on top of me and face towards my feet"

Lisa did what he asked and immediately assaulted his toes with her tongue and even though he was feeling extra sensitive he pulled down her pyjama bottoms so her cute little butt was in full view. He would of toyed with her orifices with his cock but because it was not recharged yet, a long slender finger or two would suffice.

"That's it Lisa" he sighed. "Keep sniffing and licking my stinky feet"

He then gently slipped a finger inside her entrance which earned a deep moan as she sucked on his big toe. He pushed it in as deep as it would go and slowly swirled it inside making her move her hips back and forth.

"Does that feel good?" he grinned.

With no response because she still busy slobbering over his feet, he inserted a second finger and began plunging them in and out causing her to tighten her internal muscles. Lisa's moans became more frantic as Lincoln increased the pace.

"It…It's happening!" she whined.

He could feel her whole body tremble uncontrollably and it was only when she collapsed on top of his legs that he slowed the pace down, eventually ceasing all movement. Lisa rested her cheek against his saliva coated toes as she tried to catch her breath, she twitched and whimpered quietly as she felt her brother's fingers slid out of her slick entrance.

"I hoped you enjoyed that as I did" Lincoln chuckled before bringing his slimy fingers up to his lips and licked them clean.

Still unable to speak whilst the waves of euphoria swept over her small frame, Lincoln just caressed her sweaty back and cute little butt until her breathing calmed. After a few more moments Lisa regained use of her body to the point where she shakily lifted herself up, pulled up her pyjama bottoms and crawled over to her older brother's welcoming arms.

"That was beyond remarkable" she whispered, curling up to his lithe frame.

His scent still assaulted her senses but now she was spent, it was more comforting than arousing.

"I'm glad Lisa, I really am" he smiled lovingly whilst stroking her hair.

"Why don't you stay with me tonight…we could tell mom and dad you had a nightmare or something?"

"Capital idea Lincoln" she replied sleepily. "I do not want to be anywhere else but in your bed with you"

He blushed at her uncharacteristic response but was so happy that he silently draped the covers over them and brought his exhausted little sister into a tight embrace, sleep quickly followed for Lisa but it took Lincoln a little longer because of the thoughts of what he wanted to do with her the next day.

"Sleep well Lisa, you're gonna need it" he grinned, kissing her forehead.