Watch with curiosity

You decide to stay where you are, your head poking in just slightly off the side of the window. You've never seen someone turn up close before, especially one who's doing it willingly.

The two women don't do much however, awkwardly sitting next to each other and trying to avoid each others gaze. Suddenly the older sister turns to the other. "Stacey, you're usually the outgoing one and, uh, the one with more experience with this sorta kinda stuff... How, uh, how do we... You know, start?"

The younger sister can't help but chuckle. "You calling me a slut?"

"What?! No! I mean, uh, well... You are a little..."

"Psh! I've only slept with like, most people I've met, hardly qualifies me as a slut," she say with a joking manner as she places her hand on her sisters shoulder, "I want my sis' to 'ave the best experience possible, so lets start slow alrite'? If that makes you comfortable."

She slowly places her eager lips on her hesitant ones for a smooch. It quickly turns racey as they passionately make out, Stacey's hands gripping the side of her sisters face; it almost looks like she's gnawing at her face now. It's clear these two, though the younger one more than the older, have wanted this for a long time and were just waiting for a good opportunity. Stacey climbs onto her, her thighs trapping her legs like a vice, pinning her down to the sofa. She slowly takes her sisters shirt off, dropping it on the floor without a care, revealing a rather flat chest, with two, small pink nipples standing erect. She kisses on one, nibbling on the extended tip, causing the older sister to moan like a woman not used to such treatment; holding back, with lots of gasps and perhaps a touch louder than she should be. Stacey grins a shark tooth smile.

"Stacey -ah!- I thought you said you'd go slow..."

Stacey almost jumps off in shock. "Oh shit sorry! I forgot you ain't a one night stand! Want me to slow down a little?"

The older sister just kisses her back. "No, keep going. I like it."

"Eheh-heh, that's my sis'!"

Stacey hastily yanks her pants off, revealing her sisters wet panties, which quickly get discarded. The older sisters snatch is a hairy mess of a muff, coal black hairs growing wildly in tangles.

"Aw sis'! You 'ave it just how I like it!" Her sister just grunts a reply, her hands tightly clutching at the cushions. Seeing this, Stacey unbuttons her flannel shirt, and quickly throws away her lacey bra. It's hard to see from your angle, but they're like two overweight, fleshy watermelons! She gently encourages her sister to suck at the teat, which with some hesitation she does so. Stacey groans, her hands holding up her heavy breast for her sister to drink from.

The older sister suddenly stops and coughs, a drop of yellowish milk dribbling down her lip to her chin. "Sis'! I feel sick- like... like... It feels strange, like I'm in heat or something!"

"No no, that's good, it means it's workin'! Don't worry sis', I'm 'ere with ya', an' I'll be 'ere with ya' as long as it takes, just keep drinkin' an' you'll feel better..."

The older sisters just grunts and nods, still continuing to suckle at her breast like a newborn. You can see she's already changing; her skin is dulling to a gray, pustules starting to pop up around her body, even her movements are different, more eager, less hesitant.

"Mmm Stacey, I wish I had breasts like yours, they're soooooo goooood! I just want to suck and suck and suck, and suuuuuuck! So goooooooood! Fuuuuuuck..." She moans gutteraly, her hands now kneading her other breast like a madwoman. Stacey smiles, but it's only half happy. Perhaps she's regretting her decision now, but you're sure she'll come to terms with turning her sister into a slutty zombie eventually.

Suddenly, the older sister looks at you with her big, ochre eyes. Shit! You hide to the side, hoping that perhaps she didn't see you. You doubt you'll have a better opportunity to run than now. Best not to tempt the fates, right? On the other hand, if she didn't see you, she'll probably start fucking her sister proper, and as much as it feels wrong to see two infected women fuck each other senseless, it makes you feel quite horny and excited; even from just thinking about it.