Your girlfriend's kid sister

"Hey, babe," you say as you enter your apartment. You drop your bag near the door and hang your keys on the hook. Walking into the living room, you see your girlfriend of three years, Nicole, a beautiful 25 year old natural redhead with green eyes and lovely freckles that spread across her face and down to her ample chest. She's sitting on the couch with another girl, her younger sister Sara, freshly 18 and just graduated High School and enrolled as a freshman in the nearby University where you do your research. Sara is naturally blonde, but her hair right now is bright electric blue and styled in a punk cut. She's both shorter and slimmer than her older sister, but her massive G cup breasts dwarf your girlfriend's sizable DDs.

Both girls are crying when you enter, and when they look up at you, you see Sara's always impeccable makeup is running down her pale cheeks. Her lipstick is bright blue, to match her hair. Nicole never wears makeup except on special occasions.

"Hi Sara. What's going on, is everyone okay?"

Nicole speaks up: "She's got it, babe. Sara's sick."

"What, what do you mean?"

Nicole began crying again, and Sara held up her medical report.

"I've got it, John. I'm a cum addict. My life is fucking over! I might as well drop out of school and start sucking dick for a living, because that's all I'll ever be! Just some stupid whore, blowing random guys for five bucks a pop," Sara said.

You'd be lying if you said you'd never fantasized about Sara--of course you have, she's objectively gorgeous--but when she says this, you feel your dick stiffen in your jeans.

"Listen, Sara, it doesn't have to be like that!" you say. "You're in the right city. My team is making progress in our research every single day. We will find a cure for this. And in the meantime, you won't be sucking off strangers in the street! There's a donor program through the University! I know these guys! We vet everyone who applies, and most guys get turned down to be a donor. You have to meet a very exacting criteria."

Sara grimaces, "I still don't like the idea of having to order a stranger off an app to come by to jerk off in my mouth! It's gross and humiliating!"

At this, Nicole perks up. "But it doesn't have to be a stranger! Right, baby? Aren't you signed up for the donor program?"

"Uh, I am, yeah. But just for emergency situations. If a donor is in a committed relationship, they get put on the Emergency List. I won't be out there taking calls."

Nicole continues, enthusiastically, "No, but it's perfect! Sara, you know John and you trust him. Hell, he's practically family already. You could move in with us and John could be your donor until they find a cure!"

Sara wipes away a tear. "Really, John? You'd do that for me?"

You look at Nicole for approval, she's smiling and nodding.

"Of course, Sara! I'd do anything for you."

At this, she jumps off the couch and runs up to hug you, giving you your first glimpse of her outfit: she's wearing a black tank top that's painted on over her giant jiggling tits, it rides high, revealing her tight 18-year old stomach, a short plaid miniskirt, and she's perched on 4-inch open toe heels, complete with blue toenail polish. *Fuck, you wish Nicole would take a page out of her sister's style book...*

She mashes her jugs against you and you know she can feel your erection throbbing against her stomach.

"Thank you, John! Thank you so so much!" she squeals as she plants a kiss on your cheek.