Kevin's cousin Fuller is a bit of a one-trick pony. His trick? He wets the bed.
KEVIN: I don't wanna sleep with Fuller. You know him; he wets the bed. He'll pee all over me, I know it.
A true son of Uncle Frank, we see Fuller downing Pepsi in the kitchen, and leering at Kevin knowingly. It's going be a soggy night if he winds up sharing the bed—what an unpleasant prospect.
Fortunately, Kevin gets banished to the attic bedroom, so he doesn't get soaked in Fuller's urine. The bad news is, he ends up isolated from the rest of the family, which allows them to forget about him when they leave.
So, it's a trade off. To get peed on, or not peed on…that's a question even Hamlet never had to deal with.