Maggie, Lisa, and Bart by Imouto Kitten.

Lisa was just about to crawl into bed for the night when she heard a 
tiny knock at her bedroom door. Going to investigate, the hall initially 
seems empty, but feeling a tug on the hem of her teal nightie, Lisa 
glances down to spot her baby sister, Maggie, the younger sibling 
standing there in nothing but a white diaper.

"Maggie!" exclaims the older girl, "What are you doing out of your crib 
and where did your onesie go?"

Unable to reply verbally and not giving Lisa a chance to pick her up, 
Maggie places both of her tiny hands on Lisa's knee, and leaning on her 
big sister with all her weight, manages to nudge the much older and 
taller girl back into her bedroom, stumbling, but avoiding a spill 
thanks to her grip on Lisa's knee.

Regaining what little balance she can manage, Maggie repeats her display 
of strength to shut the door, leaving the pair of sisters with as much 
privacy as they can get.

As the two girls lock eyes, Lisa asks again, "Why are you out of your 
crib?" and in lieu of a verbal answer, Maggie puts her hands on her hips 
and undoes the tapes, letting her, thankfully clean, diaper fall to the 
carpet. There is a moment of silence before Lisa exclaims, "When did you 
get that?!" For you see, protruding from just above Maggie's tiny, 
toddler twatty stood a bright yellow and clearly erect penis.

Maggie's member wasn't that big in absolute terms, maybe five inches 
long by Lisa's estimate and barely thick enough Lisa wouldn't be able to 
touch the tips of her thumb and forefinger if she wrapped a hand around 
it, but on the toddler's tiny body, it looked huge, easily longer than 
any segment of her limbs and thicker than her thigh. Lisa didn't change 
Maggie's diapers that often, but her baby sister had certainly been a 
normal girl the last time she had, so after another stretch of silence, 
the older sister asks, "Maggie, when did you grow a penis?"

Maggie had no idea where her new bit of anatomy came from, but with all 
the zany things that happen in and around Springfield, the toddler 
wasn't going to question it, nor was she going to question the urges 
that had come with her transformation. Shrugging her shoulders, Maggie 
extends a finger and jabs it towards the ground before crossing her arms 
and tapping a foot in expectation that her big sister will be smart 
enough to understand the gesture.

"Huh?" replies Lisa, making Maggie scowl like their mother and cluck her 
pacifier once in annoyance before dropping to her hands and knees. "Oh, 
you want me to get on all fours?"

As Maggie climbs to shakey feet and nods, Lisa gets down on her hands 
and knees instead, confused by the situation but hoping that playing 
along with the toddler's requests will eventually satisfy her curiosity.

Smiling around her pacifier, Maggie toddles over to stand behind her big 
sister, flipping up Lisa's nightie to knead the older girl's plump rump 
through the soft fabric of white cotton undies, though not for long as 
she soon reaches for Lisa's waistband and yanks with all her strength.

"Maggie, what are you-" starts Lisa, only to be cutoff by a cry of mixed 
surprise, pain, and pleasure as Maggie mercilessly buries her massive 
member cervix deep in Lisa's luscious lovehole, popping her big sister's 
cherry without a thought. Maggie doesn't give Lisa time to recover from 
the sudden deflowering and is soon rutting the older girl like a bitch 
in heat