Someone here recently gave good advice on how family members should support sks.
Said family members, they said, should offer an ear, words of support, advice and/or a shoulder to cry on, and then butt out. That includes situations where SK has a problem with sp. To all aunts, uncles, grandparents and BMs of sks, if sks come to you about problems with Dad/sm, offer them your love, support, advice and then butt out. You as family can and should be giving them those things. You should not be butting into affairs that don't involve you.
This happened to me on a smaller scale years ago. The sks didn't tell bm the entire truth about some things they did. Bm called dh and called him vile names. Instead, she should have offered SDs her love, advice and support, and asked them if they had any blame in this, and what they could do in the future to prevent it from reoccurring instead. Her butting in made a bad situation worse.
I can understand sks looking to family for love and support in situations like these. But some of these relatives need to butt out after offering the sks their love, advice and support.