Assuming that your SIL didn't abuse your brother in any way, and we're just talking about a run-of-the-mill divorce, I don't think you are under any obligation to cut ties with your SIL. You can still maintain your independent relationship with her as long as you are respectful of your brother (personally, I would just make your brother/the divorce off-limits for conversation) and be sensitive--don't invite her to extended family gatherings if your brother isn't comfortable, etc. But you're right--she's your friend, too, and the mother of your nephews.

Definitely don't end the relationship if it can be salvaged. Keeping the relationship is what is best for your nephews, and hopefully your brother is mature enough to understand that. It's much better for the kids if they feel that the people they love all get along adequately.

What you do depends on how often you see her. Keep her on your holiday card list, stay Facebook friends, etc. Once the divorce stuff blows over, you can be in a little more contact. But keep things light. If divorce issues come up, say it's better that the two of you not talk about it. Tell her you value her friendship, think she is a great mom, whatever nice things you can come up with, and say you are glad you can stay in touch despite the divorce.

My brother and SIL are headed for divorce and I just don't know what I am supposed to do about my relationship with my SIL. She and I have become friends over the 13+ years that they have been together and are in much closer communication than my brother and I. It makes me sad to know that she will no longer be a part of family get togethers, though I accept that as part of life. But what do people do in these cases? A part of me feels that maybe I am supposed to end my relationship with her in solidarity with my brother. But the other part of me feels that I should also value and respect the relationship that I have with her directly and I worry that she may be my only real link to my nephews.

What do people do in this type of situation? Note that I fear that the divorce is starting to get a bit ugly, for which I think they both hold their share of responsibility.