My brother divorced himself from his family for about 7 years (yes, we did contribute to the situation but it was not divorce-worthy IMHO). He really hurt our mother the worst. He has been back in the picture for a while and mother is really happy about it. My sister and I are cautious. We are keeping a low profile and do not get entangled in any of the mess. Families is why I love my friends. At least I had a say in picking them.
Hope the relationship with your SIL keeps going strong.