Your little sister bounds over and bumps Meg out of the way, before beginning to rub the head of her dick against your dribbling hole. She rubs as much of the slowly leaking semen onto her cock as she can, before pressing lightly against your sphincter. "I always wanted to try this, Doug! Everybody says it feels really good, but all the boys at school are too scared to let me put it in! They kept saying I was too big, but I know you can take it, right?" The pressure on your sphincter intensifies, and the abused muscle simply gives out, letting Sarah's fat dick begin to force its way inside.
You can feel how thick her rod is, and try to beg her to stop, but with Mandy's cum sticking to your throat and her cock still stuffed in your mouth, you can't do much more than warble something incoherent. Mandy hums contentedly as you try to talk around her sticky shaft, and a little more of her cum oozes onto your tongue. "Ohhh, that's really nice. C'mon, I think I've got one more in me!" Mandy's slender hips begin to rock back and forth again, her hands gripping the sides of your head to keep her balance. This whole situation seems somehow familiar.
Sarah slowly works her way inside, stretching your anus wide. Despite the lubrication, you still feel a distinct burning sensation, and briefly wonder if she wasn't simply scraping Meg's cum off of your intestinal wall. When she finally bottoms out, you feel her dick throb against your prostate, and she grins at you- not that you can see it with Mandy still occupying your mouth, of course.
"I'm finally in your ass, Doug. You're the first boy I've ever been with, you know? And they say it always hurts the first time, so, I figured if Meg got you first, then you could really enjoy it with me..." You want desperately to correct her, but Mandy's begun to pick up the pace, and the only sound coming from your mouth is a wet slurping noise as you try to suck down air around her thrusting shaft.
Sarah slowly drags her thick member out of your ass, until only the head remains inside, then proceeds to drive it back into your depths. She groans, and starts making short, quick thrusts into your ass, jabbing your prostate with every stroke of her cock. Though you're in a fair amount of pain, your own member has risen to the occasion, bouncing each time your little sister's balls slap against your ass.
Mandy twitches in your mouth again, and begins to spurt once more, pressing herself against your face yet again. It's much thinner than last time, and trickles down your throat, helping to clear out the remains of her previous orgasm.