Assalamu Alaikum
It seems that you have some family issues, where your mother is very angry with your father and she wants you to feel the same towards him as well. Everything she says about him she probably thinks he deserves. So, even though you have reminded her, she still doesn't care. You don't have to backbite your father, nor be a part of the backbiting, so I don't think your mother is angry at you because you don't backbite him but because she feels you are taking his side over hers and defending him. So my advice is don't get involved. Respect both of your parents the way you should, and if your mom backbites your dad in your presence just leave the room or tell her to not bring up his name in your presence. You don't have to be rude about it or anything, but don't let her make a habit out of talking about him in front of you. I know it sounds crazy but she probably backbites him because she has no one else to rant to or discuss her marital problems with, so she does it in front of you.