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Why do people think poly=no boundaries?
I know of very very few poly people who would be okay with that. And even if they were, poly requires open communication, and he did it behind her back (which we know she wasn’t okay with it because it ended their relationship). So a double whammy.
Highly recommend the book Opening Up to understand how many different types of poly there are and what they might look like.
I get that and I appreciate how respectful you’re being. I think we can tell by the subtext that it wasn’t okay to sleep with her sister (they broke up over this, after all)… and also that is kinda soft incest (to share a sex partner between siblings) so majority of poly people in the west would really not be okay with it due to social norms.