Mom wants back into my life

I'm 18 and my mom had me when she was 17 and my dad was 18. My dad has autism and he received a disability check monthly so that's one way they were bringing in income. When I was 6 my mom kind of decided she didn't want to be a mom anymore and left. So me and my dad went to live with my grandfather and his wife. The last time I heard from my mom was almost 12 years ago. She reached out to me. For a favor mainly. See she reached out to my dad first. She has been to therapy and realize that she was happiest when she was with him and reached out to him to apologize for everything and said she wants to be together after not speaking to him or us for almost 12 years. Well my dad is currently engaged to my soon to be step mom anf he obviously turned her down. After talking to my dad he told me my mom offered to see him on the side to give him a chance to fall in love with her again behind my step mom's back. The reason my mom reached out to me is because she wanted me to talk to my dad to convince him to give her another chance. I said no and she starts saying how she regrets breaking up the family and she has gotten the mental help she needs and wants to put our family back together and needs my help to do that. I again said I am not convincing my dad to leave my step mom who I do adore because she's coming back over a decade later. She has continuously been begging me to talk to my dad. Sabotage their relationship , etc and telling me all we can do together once the family is back together. But why can't she just try to rebuild a relationship with me? Why does she need my dad back in order to want a relationship with me. She "apologized" for abandoning me but follows it with why she needs me to do this for her. Honestly I don't see any good coming out of having her in my life at this point. I don't even think she would have reached out to me of my dad agreed to cheat with her.