Is it Wrong to Wear a Thong to A Family Pool Party?

My big sister threw a July 4th backyard pool party, and I came with my girlfriend. The problem was my girlfriend chose to wear a thong-style bikini. Everyone seemed cool about it, except sis. She took my girlfriend aside and scolded her for her "inappropriate attire," saying she ought to be mindful of the fact that there were children running around, as well as her husband and teenage sons.

Well, my girlfriend wasn't having it. She tells my sister, hey, there's no reason to be "ashamed of having a mom body," and it was nobody's fault if her husband can't keep his eyes where they belong. And it went downhill from there. Now my girlfriend is mad at ME, saying I let my sister disrespect her. And my sister called to tell me not to bother bringing that "trailer trash" back to her house. Who is right? More importantly, how can I mend fences?

Is it Wrong to Wear a Thong to A Family Pool Party?
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