My exH cheated on me for over 20 years and three DC before I belatedly wised up. His speciality was enlisting our mutual "friends" to keep his secrets. Not for nothing was Icarus his online name on the OLD sites where he recruited women :)
My take on it is this. I understand that people fall out of love. If he had said this to me I would not have been happy, but ultimately I was 28 with a great career when we got married and if he had fessed up I would have picked up my life and probably met another man who did actually like me and had DC with him instead.
What he actually chose to do was lie to me over a period of years, father three DC and secretly resent me for trapping him (unbeknownst to me) so he picked away at me, emotionally abused me and fucked me up over the 20+ years we were married.
It's at my door that I had such low expectations of a relationship that I stayed all those years and wasted my life. I've had a ton of counselling and I understand now why that was, but that does not alter what he did.
It does not alter the fact that those women all knew about me and the DC and chose to have affairs with him. It does not alter the fact that he married one of them and they are still married today 6 years later.
I don't really care if their relationship is successful or not. Neither of them had any morals. The fact that they are still married does not alter their behaviour any more than the fact Charles ultimately married Camilla alters the fact that they fucked around while he was married to Diana. We are supposed to forget that in a 1984 kind of way, but we don't. Getting married does not whitewash your behaviour.
If my exH had been a decent man he would have come to me when he first felt attracted to someone else and said he wanted out. In fact he was a bratty, nasrcissistic man child who wanted the appearance of being a family man whilst shagging prostitutes, co-workers and anyone else who would have him and calling his affairs "love" because in his eyes that was the ticket which made him blameless.
If the OW he married had been a decent woman she would have refused him until we were divorced.
I haven't RTFT in sufficient detail to follow your timeline OP, but I suspect it was a bit murkier than that.
So ultimately, your SIL doesn't like you because you were OW and takes every chance to let you know it and that she holds a grudge. That is her prerogative. Yours is to avoid her company.