I was one whose SIL was very supportive for a long time; we are still on good terms (I think), however as she (more to the point, her H) has decided to accept OW (her words "I'm trying to get used to it") and they socialise with H and OW, I no longer call. And she doesn't call me.
I supported SIL through the death of their mother, there is a piece of me that feels that I was only kept round because I was useful for that (and even SIL said that H was useless), but then I thought about it and realised that I would have done so anyway. I was so glad that I could be with my MIL in her final hours.
So my parents-in-law have now both passed, and in the end my r with SIL is now petering out as well. She has always said that I am family, but since this latest OW has been 'accepted', I won't attend anything where OW goes, so de facto I'm not there any more. I did explain to her, so it's not like I just disappeared.
And my kids point blank refuse to be anywhere OW is.
SIL is trying to walk a tightrope, I understand that she's doing what she feels she needs to as a sister, but my kids right now aren't clamouring to see her either, although none of us could turn our backs. So we'll see.
I've heard all the 'usual' things from friends and other family, my brother is wonderfully supportive, though, even though he deplore's H's actions. But he supports ME, and that is what counts.
Friends are more and more seeing my point -- in the beginning I heard all the usual "move on" stuff, now they see that that doesn't solve anything, and perhaps they also don't see me as a threat to their own marriages, so that is all good again. They get what I say and what I'm doing.
We'll see what happens with SIL; I'm opting just to be patient and trust that it will all turn out as it should.