Craig is Tweek's hypersexual (step)sis and masturbates constantly and it drives the whole house crazy. Tweek wakes up with his sister riding his cock one morning and she's degrading him and telling him he's basically a girl anyway and sticks her fingers up his ass to make him cum (they're both gay). Really funny if she's all "Great now I'm pregnant and our baby's gonna be deformed," and they're not even related. But it's all up to you