A wise woman (OK my Mom), used to tell me "you can't get blood from a stone". You also can't get love from those unable, or unwilling to give it. We don't choose our families, and although we may have to "love each other", it doesn't necessarily mean we like each other. For a mother to feel this way about a daughter is unfathomable to me, as is playing favorites, but parenthood is not a ticket to sainthood.
My own sister and I don't like each other, don't respect each other, and have no relationship. I don't agree with the way she lives, she doesn't agree with my life choices. Fortunately our Mom (a single parent), loves and likes us both, although her relationship with each of us is different, and I don't agree with the way she lets my sister treat her. I learned long ago that any sort of relationship with my sister beyond civility is not possible, and to be honest, I think it's better for everyone. I, like you, have two small children, and decided my own family is my priority. I can't worry about my sister, or my Mom for that matter. They're grown women, with their own lives to live, and I have to worry about making a life for my kids. The freedom of that choice is enormous, let me tell you! I used to get angry, or frustrated with them both, now I just say "whatever".
For your own sanity, stop the cycle of hurt. You can't control anyone elses behavior or attitude, but most certainly can control your own. And always remember, those that give love freely, deserve to receive love freely. Good luck.

P.S.
I did let them both know how I felt before I "wrote them off".