When this uncle is conflicted, he asks Reddit:
I 37(M) and My wife 34 (F) recently had my older sister and her kids fly out to meet us my oldest nephew is 18-20 I won't list his age for privacy is a bright kid and is ridiculously intelligent for his age.
The issue arose when I took them and my kids to a waterpark My nephew was wearing in a loose fitting activewear shirt and swimming shorts.
I told him that he wasn't dressed appropriately for swimming and he stated that if he could complete the BSA swim test in what he was wearing it was good enough. For context my nephew is very fit and lifts and exercises regularly.
The issue arose when I stated that he was being weird by not going shirtless to a waterpark and that he should change into something more appropriate he reiterated his point and said that If I was going to force him to change he wouldn't go.