My brother and I had a a terrible relationship growing up, even in our 20's we couldn't be in the same room together longer than 5 minutes before we would start arguing with each other. Just by chance, we discovered that our mother, thinking she was "helping" us with our relationship, was actually the reason it was so bad. We got along after that but it wasn't until our mom passed away that we became really close.
So it was especially hard when my SIL turned her back on us, although it really wasn't out of character for her to do so. And my brother isn't a strong person so I wasn't surprised when he just accepted it.
So now I'm just hoping we will get back on an even keel. It really helps that my DH totally understands, probably better than my brother and I ever will, since he's known her all his life.