Why does my mom do this?

Obviously y’all aren’t going to know all the answers but she doesn’t answer me so it’s confusing. So I’m 24 and my mom and I have been close most of the time. My sister and I don’t get along as she was talking to my ex husband and I why we were together and slept with him. She has always been in and out of my moms life and rude to her to be honest. She always argued my mom did the most for me but it was just me getting along with her and not arguing with her when we were teenagers. Fast forward we are 23 and 24 now and I divorced my ex husband 2 years ago. My mom started getting closer with my sister and I told her it’s fine but I don’t want her involved around my new relationship. I even go eat with my mom and she’s there and I don’t cause problems if we are in the same room. I even watched her kid. My mom ask why I don’t want to be close with my sister and she says “it’s the past you need to move past it.” Yes it’s the past I agree but I can’t just forget about my own family in bed with my husband? Like who does that?. My real problem here is that my mom will drop me if she’s close to my sister. She will hardly call and this and that. If she’s not speaking to her or complains about her she will call me and want to hang out. I’m her daughter though and starting to feel like her rebound in a friendship at this point. Like I don’t even cause problems with my sister when we all go places because I try to respect my mom is also her mom of course. My mom though just stops calling and a few times we have stoped talking completely over her just being close to her again. Why? And what kind of mom does that to her daughter?. Just because it’s family doesn’t mean you put up with toxic behavior from them. It’s the same way the other way around as well though if she starts talking to me then she won’t her as much. It’s confusing all around to be honest and hard because I have always been close to my mom. What to do?