Kind of a stupid question, but what kind of bathing suit should a girl to wear to a pool party at 13?

13 is kinda young to be wearing anything "risk-ay" so to speak, so if you go with a bikini, I would recommend that you buy something that covers more than reveals. A string bikini would just be inappropriate. Since you don't want to offend any family members, your number one safe shot is a one piece. Although no one said the one piece had to be ugly. Best of luck to you.