At odds!

At this current point in time, I feel as though my mom is literally choosing her relationship over everyone. I know she isn't the most secure and confident person in the world but she was a happy, cheerful and pleasant person to be around until her current "relationship" if you even want to call it that. He has done nothing but belittle her, talk to her as if she's dumb, use her (financially), etc. He as a person isn't all that bad but as a partner, he is complete poo. (I'd like to use another term but you get what I mean.) He takes advantage of everything that she has been through and she lets it happen. I just don't understand it. I say that to say this... I speak for both my sister and I when I say, that our relationship with our mom is getting worse and worse. My sister has her own place with her kids but I have to stay and deal with this until I am able to move out on my own. I have no idea as to what I could do to make (my) relationship with my mom better. She's the only parent I have left and I definitely don't want our relationship to turn out like hers did with her own mother. Obviously I'm completely desperate / have no one to talk to about this who I feel truly understands what I'm going through as I'm pouring out personal info on an app... But has anyone gone through this? Where your mom has relationships with people that you just don't understand? Or have such a bad relationships that you wish to make better?