My mom left me, what should i do!?!?

okay so ima fifteen year old girl, last year both my parents got cancer at the same time and it was hard on everyone...both my parents are fine now but a month or so after my mom recovered she changed so much, my mom picked me up from my boyfriends house one day and met his moms boyfriend..within the next few weeks my mom wanted a divorse from my dad, i accepted it because theres not much i can do, my mom left with my little (seven yrld) sister to go to my aunts house, i chose to stay with my dad for the time being because i dont get along with my aunt. a few days after that my mother calls me and tells me she is involved with someone when i asked her who she said it was my boyfriends moms boyfriend! not only is that man discusingg and a child molester hes a druggie and alcholic, i freaked on my mom and told her never to talk to me again, as the man has touched my *** and my thighs and my mom was aware. now seven months later i have only spoken to my mom threee times, there going through court for coustdy and my mom doesnt belive a word of anythingg i have to say anymore. i really miss my mom but i dont want to have a relationship with her as long as shes with this man...what should i do???