Toxic relationship with my mother

For years I’ve had a very toxic relationship with my mother. It started when I decided to live with my father at 10 and she told me she was no longer my mother that my step mom would be my mother now and I wasn’t allowed to call her. It got worse as I was sexually assaulted as a young teenager and she told me it was because of the way I dressed with so much more in and out. All of these things I was willing to look aside for my children to have their grandmother. Recently my step siblings were taken away by CPS for sexual adult from my step father and my mother decided to stay with him and abandon the children she raised from babies. I told her I was trying to support her even when I didn’t support her choices and tried to keep the door open until now. Now my step sister would like to come live with us, which we are very excited and nervous about. My mother didn’t let us spend much time together. My mother freaked out and forbade me from taking her in from the group home. I already have my foster care license and room in our heart and home for her so for us it wasn’t an option to turn her away. It’s not come to light from voice recordings my sister made to protect herself the sheer verbal abuse she suffered from my mother like I myself used to. What hurt the most was hearing that she didn’t like my two older adopted children but had to be okay with it because that’s the only way I’d let her see my youngest and only bio child. I suspect it has to do with the color of my children’s skin due to other racist tendencies she shows. So here’s the question, I’m pretty sure this is the point where it’s too toxic to continue. But how do I cut her out? And why do I feel so sad about it given that I’ve not had a personal relationship with her in years?