How do I cope?

My mother has always shown favoritism towards my brother. Then it was to his son, and finally his grand-daughter. I had learned to except this and still maintain a very good, trusting relationship in my adult years. My step father whom I was very close to passed away recently, and my mother became very very depressed. I was her confidant with all things emotionally and financially in my adult years. I was that come to person for every thing. I had finally in adult hood gain that long yearning relationship with her that I didn't have as a child. But when my step-father passed, again I was the only one that stepped up to help her. I do live a fairly distant drive from her and unable to be there for every thing she needs on a spur of the moment, like taking her every where she want when she wants. My brother is there to do and can fill that void. But the problem that has risen, my brother's son's live in girlfriend has also stepped upto help over the last couple of years, and I was so grateful for her help, but now after the recent loss of my step-dad she has been making comments and what ever else to have gotten my mother to completely turn on me. My mother talks horribly to me, and takes her side for everything. It's if she doesn't trust me, and plots with her behind my back. I would love to get my mother help for her depression but she now thinks I am wanting to hurt her for what ever reason I don't know and know it would just put more stress on a stressful relationship. I have no clue why she has turned on me and now hurts me with every word that comes out of her mouth any time I don't agree with what the girlfriend is doing. The question I have is do I step away and let this woman( grandson's girlfriend) take over and stop allowing my mother to hurt me any more with her abusive words and actions, or I'm at a lost. Please help. Also now a lot of things have come up about a will for my mother by the girlfriend and nephew( my brother's son). She again is partial to him and his daughter over the rest of the grandkids. I'm hurt depressed and at a loss for what's best for my mother's well being and also my well being. Please help. If I say anything it only makes thing worse. Hurting, and I don't know why this is happening. I've lost my mother's friendship, my mother's love, and I have no ideal why.