How do I deal with my mother and brother who are on the outs? Mom is 70. Brother and I are 50ish.

My brother and mother had a falling out almost 2 years ago. My brother instigated it and mother made it worse. They didn't speak for a long time and now are barely civil. Brother is newly remarried and everything was great then but I feel my new sister-in-law is manipulating him to estrange himself from his family. (she has estranged herself from her own family) She also did the same thing to her last husband. I would like to see everyone apologize for their part but both mother and brother think they are right and won't budge. I'm so mad at both of them but at the same time feel sorry for them. Mom is getting older and is depressed and I'm not sure how to help her. All I ever hear from her is how horrible my brother and sis-in-law are. He's still my one and only sibling. Our father is dead so no help there. I got in the middle of it when if first happened and now brother is cold to me, too. He threatened to "divorce" me. I tried apologizing but he was cold to that, too. I get to the point where I want to write them all off. I feel brother has ditched his responsibility to his mother and left it all to me. I resent him for that and resent mother for not doing whatever it takes to have a relationship with her son. Help!