i DON'T want a relationship with my MOM!!!

this will be long I am so sorry. i need some advice... my mom and i have never been close. she allowed my step brother to sexually molest me when i was a child because she didnt want my step dad to leave her. needless to say I do not have communication with her and she has missed out on ALOT on my life and my children's lives. i moved to another state at the age of 15 to get away from her and my step brother. so recently my sister who is 19 reached out to me (who hates me btw, and allows my mom to fill ideas in her head about me) and asked if she could spend some time with me and my 3 children. of course i said yes however my boundary was that we would go to Counceling to work thru our issues because i was not going to engage in any negativity. so we end up going to Counceling and the whole situation with my step brother and mom come up and my sister is shocked. she had no idea this had happened and understands why i am the way i am. she spemds three weeks with me and we have a wonderful time. we got to know each other again and i can honestly say she is the sister i had always wanted but never knew i had! so fast forward to the day she leaves and she tells me that she wants me and mom to work thru our issues and be friends just like she and i were able to. i tell her no kindly and explain that i am not interested. the next day my mom calls and says that my little sister told her that i wanted to talk to her about something important. i deny it and tell her that she was tripping and hang up the phone. well now my mom is curious af to know what i "wanted to talk to her about" and keeps calling me!! i call my sister and ask her why she would do that and says that she thinks its a good idea we finally hash everything out between us and squash it. i am so hurt by her because i have to revictimise myself again talking to that woman. i have done it in the past and she told me in my face that i was a liar and that if my step brother really molested me i would have ran out of the house kuz thats what she would have done in my place. how do i make my sister understand that i am not interested in a relationship with her but not sound rude since we just repaired our relationship. i already told her that the reason i stay away is because i have some sense of control over the situation this way. the power that was taken away from me at 8 years old was regained when i moved away. i choose what i want her to know and partake in my life not because i am doing it as payback but because it gives me a sense of control and power over the situation. idk how to explain it to her in terms that a 19yr old can understand but also be sensitive and not loose our relationship that means so much. please help