Yes, it's okay to feel this way ... And while you don't blame her for what she did (and didn't do), I certainly blame my father for knowing how badly I was being abused by my mother and older siblings and CHOOSING TO NOT take action on my behalf!
It doesn't matter if the other parent is being abused too or not ... As the other responsible (or even irresponsible) adult in a child's life, it's their job to get themselves and their children out of those living hell's ... They certainly know right from wrong and there's no excuse for NOT PROTECTING ONES CHILDREN!
If they choose to not do this, then to me they are JUST AS GUILTY as the abuser because basically they are sacrificing the physical, emotional and spiritual well-being of their children on the altar of the abuser's depravity.
That's how Pfrog sees it, and I ain't cuttin' nobody no slack for failing me! ... An innocent vulnerable child that couldn't protect or defend herself!