need some advice...no judgment please

I was molested at age 10-20 by my dad. When he deemed me old enough he tool my virginity. I havent told many people about it. I thought I could just forgive him and then stay away from him. But he and my mom now have children that they are fostering and hoping to adopt. my mom knows of what he has done and threatened if I tell anyone she will say its a lie. I now feel responsible if something happened to the children because 2 of them are little girls. What should I do? If I report him to the police it will tear my entire family apart....but if I dont and he does something to one or both of the girls then it would be partially my fault...these girls are only 3 and 5