You use teamwork!

“Kary, let’s do this! Stop stuffing your face!”

Kary crams the last bag of chips into her mouth and puts her hands on her hips. “Mmrph,” she says, and starts thrusting.

You start to open for her further, and little by little, you swallow her cock. You needed this so bad; your sister’s dick in you feels so right, so natural. When she finally pops the head in, you slide back all the way until you are bottomed out against her crotch. Her cock bulges out of your belly, which has gotten used to being stretched and feels perfectly natural filled with Kary’s dick.

“Kary, want to cum in me, the proper way this time?”

“My virgin dick is in my brother’s big butt...of course! I’m gonna knock you up!”