Quite often, someone will place themselves in front of an abuser to protect someone else--whether it's a parent protecting his or her children, or a sibling protecting younger siblings. They think that they are protecting the innocent from being hurt.
Is that really the case? It's only a temporary solution from physical or sexual harm. The children still soak up the abuse emotionally. Children either learn to abuse, or to cower in fear. They learn to lie, to pick on others, to not stand up for themselves, etc.
These victims can't protect others from their abuser by offering themselves as bait. Abusers usually will go after other close targets. Really, a victim is only prolonging the inevitable. A promise from an abuser means nothing when it comes to protecting someone else. A victim can only trust that an abuser will most likely abuse again.
An abuser has lost all trust. They do not deserve to be trusted again. The victim can forgive, but never trust the abuser again.
The only way to protect others is to get out of the situation if the victim can. The victim can pray for help, and ask others for help. I've talked more about escaping abusers here.