Should I go to the beach with my Hypocrite Sister-in-law?

all that you said at the end of the last paragraph, say it to your boyfriend. better yet, confront her about it. tell her you know and think that thats horrible because you havent been anything but nice. if nothing comes of it, dont help them out. simple as that. you seem like a very mature and sincere person, dont expect everyone to be like you and you shouldnt stoop down to their level either, but putting your foot down should let her know you have limits too.