My mom's boyfriend molested my sister, my mom doesn't believe her.

A couple months ago, my sister told me she had been molested by our mothers boyfriend. he was living with us by this time. I was getting ready for bed it was about 11 oclock at night. Me and my sister share a room, shes 11. I could tell something was really wrong so i asked her what was wrong. She said it didn't matter and to just leave her alone. I could tell something was obviously very wrong and i wasn't going to leave it alone. Every time i tried to get it out of her she got more upset. Until she finally said okay, but i don't know how to say this.. I said its okay I'm here for you for whatever it is. we have a very close relationship. She said chris touched me on my boob. I was so shocked, and asked her if she was sure, i was trying so hard to hold back the tears and i had no idea what i could do. I sat on her bed with her and hugged her. I said i think we have to tell mom. She immediately said no and that she couldn't do it and that our mom wouldn't believe her. We sat there for a while and i knew what i had to do. I went to the living room, my mom was watching tv, her boyfriend was asleep on the couch. I said mom me and maddie have to talk to you, with tears streaming down my face. My mom jumped up and came to our bedroom wondering what was wrong. I think by the way me and my sister were hysterically crying she guessed and asked my sister if she was molested. My sister shook her head, my mom said who!! And i answered and said chris! My mom immediately started crying. A lot happened the rest of that night my mom had no idea what to do. She kicked him out that night. A couple days went by and nothing else had been done. It was talked about a lot and everyone was trying to figure the whole situation out. My dad eventually told the cops and thats really when my mom flipped out. She didnt know why my dad would ever do that and why he would go to the cops so soon. It was 5 days later! After that point i could tell the direction my mom was starting to go. This was in september. It is now february and nothing has been resolved. He has a court date coming up soon, i dont know when. My mom has had so many outburst and many times she has blamed this whole situation on my sister and me. I don't know where to go from here, she has said everytime he touched her it was only on her boob and then she would move and make noises and he would stop. she also told me she doesnt know if anything happened while she was asleep, she is really scared my poor sister has fought so hard to get my mom to understand but my mom will say it didn't happen!! Its made up! You were dreaming! Anything to try and get my sister to confess. She believes my sister is bipolar and that is the reason for everything. My sister was attached to her boyfriend but became more and more attached toward the end of the summer, which is when it was happening. My mom says if he really did do it to her she wouldn't be so attached but i have asked her why she was so attached and she has told me because she didn't want anyone to think anything was wrong so she pretended it was okay. My mom also says she has holes in her stories when she tells about the times it has happened, i asked her myself to tell me all of the different times and they seemed to all match up to me. My mom also says that me and my sister just didn't like her boyfriend and we wanted him to leave, but everything was perfectly fine and he was living with us at that point. My sister also told my little brother (8) about it after the first time it happened, they would set up barriers of laundry baskets and toys and whatever they could find to keep him away, but they would always be moved in the morning. My brother only told this to me he wont tell my mom because he doesn't want to get involved in this big mess. Also right when my sister told me this i remembered her telling me in the summer that he would stare at her in the middle of the night and i did see him once by the laundry and she was sleeping right next to it, when i saw him, it was 4 in the morning and i thought he was just getting clothes but i remembered it because it was a weird time for him to he up but it clicked with me that that is what he was doing. When she told me that he just stared at her at night i was shocked and asked her if thats all he did was stare and she said yeah, it seemed weird but i let it go then. I just need someone on the outside to just make sense of the situation and help me decide what to do?