Hello, I am somewhat in a similar situation as you... I was also abused by my step dad & my also doesn't believe me. Its such a terrible feeling when your mom, of all people, doesn't believe you.
My step dad abused me for 8 yrs, & when I brought it up to my mom a few times she didn't believe me...then she just told me not to talk bout it every again. I blocked it out for years, when I was 21 I decided it was time for me to be happy & figure out what hppened.
So I sat down with my mom & step dad & told them. He did admit to what he did but blamed it on me & said it was my fault. Bu she sat there & heard him say it but she still denies it because "our stories don't match". So after I went to my counsellor for a few yrs, I realzied she was right
I needed to cut off my mom. It was the hardest thing I had to do. I talked to my mom on a regular basis (never about what happened of course).
That's where we differ, but still my mom doesn't believe me. I have chosen to not talk to her. I've told her that the only time I will talk to her is if she goes get proper help & to tell me she beleives me & that she's sorry.
I think she's making up excuses to not believe you. Its hard because you live with them too but it was the best decision I had ever made. But I got married & moved out so it was easier.
I thik you should definately go get help so you can talk to someone who is not your mom. I hope this helped...stay strong