Its a shame your BIL does it and encourages his son to do the same. I usually expect more maturity out of family members. Especially adults. Its really hard to never get picked on over anything. If your too normal your a "sheep" if your too weird your a freak. It's usually best to just be yourself and be confident. Bullies seem to be able to smell fear. Then know which kids they will get the best reaction from. If you deny them the reaction they don't really have a reason to do it. This is hard for kids to deal with but school is quickly over and the need to fit in fades a bit as an adult. Good Luck in swim class!!!
All mine are like that. My oldest is starting to like it more that I am not right there watching but still in the imediate area.