It took much longer for me to forgive my mother for forbidding me from talking about it than it did for me to make peace with my father's behavior.
Cyranoak's comment about it becoming the child's fault is spot on. Even if you are able to not blame her, she will blame herself if she slips up. And if you have to face repercussions for what you did because of it, that's her fault too. Using an extreme example, if she tells someone the secret and you are arrested because of what you did under the influence, it's her fault that you're in jail. That you did something illegal doesn't matter. She's destroyed her family. If she could have only kept her mouth shut, everything would have been fine. She's ruined everything.
It's a really toxic way of thinking, and it's disturbingly easy to slip into.