My mom doesn't believe me

So let me start this out with my mom married my step dad when I was three and she knew he had gone to jail as a teen for molesting his brother and sister.

He molested and raped me from the time I was 5 till I was 12 (my mom left him when I was 12) but I know she doesn't believe me because she allows my siblings (ages 11, 10, and 8) go to his house by themselves for hours at a time and I can't do anything because I'm only 15 it makes me so mad