But we don’t know the circumstances here.
The OP herself has said that SIL doesn’t want a relationship with them even though she has tried to maintain one.
Also, the brother’s partner is not the OW, she was once, but if it got to the point of them establishing wills and executors and so on it was clearly a serious relationship, regardless of people’s feelings of how that relationship came about.
If the brother didn’t have a relationship with sil before her ex died then it’s understandable that he wouldn’t seek to rebuild one after the event. In fact it’s even possible that SIL might not have wanted a relationship with her husband’s family even if the marriage hadn’t ended. Sometimes this happens also.
Nobody knows what goes on within someone else’s marriage. OP isn’t wrong for wanting to maintain a relationship with the mother of her brother’s children, but neither is the brother in the wrong for not wanting that given the brother was in a relationship with someone else at the time.
Fact is, if there weren’t children in the situation the OP may not have maintained a relationship with SIL either. I’m pretty sure that if I died my family wouldn’t seek to build a relationship with my ex as my children are old enough now to have their own independent relationships with them, and they don’t have a relationship with ex now while I’m alive. And fwiw I didn’t leave him for someone else....