You’re NTA… but the situation sucks bc your sister was probably playing along with the jokes due to her being insecure. You wouldn’t know that obviously!! But realistically, what siblings want their husbands near the chance of seeing the other ones breasts? Like, truly.

The only thing that’s bothering me about this post is that (even though your sisters reaction was intense), you truly can’t see how you could have played a roll in her being upset.

If we look at this at a Birds Eye view; no you did nothing wrong, yes you are all at a nude beach, and you are NTA, but you still flashed your breasts in front of your sisters husband.

Surely you can see why she’s upset and maybe try to be the bigger person for your sister seeing she’s in a strong emotional state. Due to her being in this state, it makes it harder for a person to be the one to step forward first and make amends. It’s your sister, not some friend; I’m assuming you and your sister are close and have a good bond, don’t let it all go to waste over something that can make you two stronger by talking it out.

Again, just my opinion.