I'm afraid to me this is still thankfully a theoretical question. Very sorry if it's upsetting; I'm sure it will be to some.
I saw both my siblings and their families this weekend - unusual, although we are all on good terms. It struck me that although my BIL and SIL are very nice to me, I can't really imagine them wanting to spend time with me if their partner weren't around, and I wondered if people ever manage to keep these relationships going.