If the mother is physically unable to contact the police, that's a different story, but I don't think fear is an excuse.
And I think the mother is just as guilty as the father, since she has the ability to end it all whenever. Not even death threats justify not telling the police about your child getting violated or abused.
By the way, I'm sorry to hear that, woof. I'm not trying to be an ass or anything, I just don't think personal experience should dictate your opinion in a debate. It's useful input, though.