ok let's recap.
op said they weren't going to go outside.. it's not like she was there and then sister and friends came later.. they were there first but assured op that she would be ok.
Anyways, yesterday my younger sister invited her friends over to go swimming in our pool. At first, I didn’t wanna go outside but I was very bored and decided I would. I changed into my bathing suit and went outside.
Since my sister’s friends are only a year younger than me, my mom warned me they would stare. However, my mom and sister agreed it was okay for me to swim when they were there.
well it turns out one of sister's friends wasn't ok. so you run into several possibilities.
simple answer (not economics) would be tell BF to stop staring at tits. that assumes he was staring at her tits. but op never said he was or wasn't. on top of that, that doesn't fix H's insecurity.
I get outside and notice my sister’s friend (let’s call her H) began to whisper something to my sister.
she explained that since H’s boyfriend was there, H was NOT comfortable with me wearing a bikini in front of him
what i'm saying is in this situation there is two options. she came in after the fact, mom had warned her it might not be ok. she arrives.. it wasn't ok. so she could
A) change and make it ok (even though she isn't wrong in wearing it, being her house and all) but the environment is deeper than her lounging at her pool with just her family around or
B) she could say no. causing H to be upset, sister to be upset, mom to be upset and in turn op to be upset (or else she wouldn't have written this post).
economics would be which a better benefit for the cost?.
op changes (medium cost) everyone happy (high benefit)
op doesnt change (low cost for op, high enjoyment cost for sis and H) and low benefit for h and sis
it's not an issue of pleasing anyone really or reprimanding a girl for her body. you can change all the players to a formal-ish event. guy tells gf to dress nice. she wears a dress and heals with a fur shawl. he assures her she'll be fine. they get there, it's a high-cost gala type party for PETA. a couple snarl at her in disgust. she could A) make it easy and remove the fur before it becomes a 'thing'. or B) wear it in pride because she can and it's her house and she has every right to. strictly speaking, yes she has every right to and isnt in the wrong.. but sometimes interactions with others are deeper than simply if you are right or wrong.