Mixed, I think. My FIL passed away about 5 years before BD, indeed, there were people who wondered if he never dealt with his father's death properly.
At BD and for years afterwards my SIL (H only has one sister) said that I was family, that I mattered, that I was important, all that. For the first few years they were exasperated with him, she would moan to me about how he wasn't doing what she needed (MIL had alzheimers), etc.
It slowly eroded, however. At first when they came to town they would stay with us, then they started staying with H. All probably around the time of OW5. That was the first one he took to meet them.
As a side, they thought she was just number two, they had no idea about any others. I think he edits what he tells them as well.
Since then it's not been great; while MIL was alive I continued to go see them, to take the children there (MIL was in a home near where they live), I had the privilege of sitting with MIL for a couple of days before she died. Needless to say, H didn't. As a matter of fact, right before the end H called me saying that his sister was a mess, that he couldn't really deal with her... so I did.
SIL and I had words earlier that year, as I didn't go to a big birthday party she had, where she had invited OW5. I told her it was a step too far, that she had no idea how much my children were hurting, etc. (It would have meant taking the children out of school, an expensive flight, all that). It was then that H started divorce proceedings. (H didn't ask to take the children himself, I should note)
But SIL was still cordial to me, I think she needed me for support as MIL was dying, which H wasn't providing.
The children and I attended the funeral, OW5 didn't.
Since then we have been cordial, but she hasn't called me -- I call her once or twice a year.
They have also accepted OW6, SIL's H once got angry at me and said "just move on"; I've explained that I have "moved on", but that I'm not a martyr, all that.
Interestingly, the children and I are going to stay with them this weekend, as we have an event to attend in this town.
We have had some discussions, those never end well. In general her stance is "he's my brother, he's all I have left, I will do whatever he wants". I did call her out on it once, saying "you say I'm so important, but you don't walk the walk" (in a more polite way, of course), and she just got angry and said that my kids were treating H badly.
I've chosen to remain cordial; I don't regret saying the truth, but it never goes anywhere.
It's partly a "blood is thicker" thing, partly that she is one of those people who just doesn't like to talk about difficult things of any kind. I didn't understand that for a long time; she has said that she wanted to be there for me, as I was for her when her first H left many years ago.
But now that H is behaving like her first H it seems OK with her. I've called her out on this as well, but of course that doesn't do anything. I'm not sure what her definition of "being there" is, but it's not the same as mine.
While MIL was still mentally here she was furious at H; had his father been alive I'm pretty sure he would have given him a piece of his mind.
SIL said that she had "said her piece", but she also took H's statement that he had been miserable for most of our marriage at face value. Her H saw more clearly at first, but he now just wants a calm life, and they can have a nice holiday where H lives so that's enough for them.
But H's cousin and wife have been my strongest support throughout, and they continue to be. That's the only other family H has in this country, others are distant cousins scattered around the world who really don't have any close connection.
So I continue to be cordial, respecting SIL's request that I not talk about her brother, but if it comes up I will say my truth.
The kids have been upset with their cousins, who happily spend time with whatever OW is around, I've explained that it's completely different for them, it's not their father, it's just an uncle who does "cool" things.
I once had to ask nephew not to show my S pictures of them all going out with OW, explaining in one sentence that my S wanted his father, he couldn't have him, and asking nephew to think what it would be like if it was HIS father. Nephew, to his credit, never did anything like that again.
My kids would like their cousins to understand, but that may not happen. Perhaps when all are much older....
Whew -- long winded. This must be weighing on my mind!