Sorry, I didn’t mean to cause offence at all, I just assumed that in a poly relationships the boundaries were open for negotiation, and that like you say open communication would be key. I didn’t know he went behind her back, I know nothing about this story other than what I read on this thread.
I get that in a conventional relationship, leaving someone for their best friend or their sibling would be a particularly heinous betrayal, but didn’t assume that that would be the same for someone in a poly relationship. I didn’t mean to offend anyone by asking and I apologise for any and all offence caused.
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.