I can only reiterate what others have said. The children are the top priority here, and must be protected.
It is very difficult for anyone not in the situation to understand why a woman doesn't leave an abusive relationship, even when there are children, and agnurse has summed it up very well.
I wonder - has anyone ever recorded the father when he is in full flow? Maybe if he was shown a video recording, or audio recording of what he sounds like, it may be a wake up call (although it does sound unlikely from what you've said) as to how terrifying it is for young children to witness.
Has anyone spoken to the children's schools about the home situation? Schools in the UK are very proactive about child protection and can implement all kinds of measures to help and support the mother and/or the children. In your situation though, it sounds as though the mother isn't ready to acknowledge the situation and seek/accept support, so it's down to the adults/professionals to take steps.
I strongly believe that the adults in this scenario need to forget about "what happens to me if I . . . . " and just do something to protect those children. "So what" if your stepdaughter "kills you" for reporting to Child Protection Services? You say - "most of his abuse is verbal abuse along with all the yelling, cursing, and complaining. You can see how its affecting these girls, he's a large intimidating man, I can only imagine the nightmares the girls must have about him, he really is a monster to them." And yet you still hesitate to do anything to safeguard them
Not knowing how things work in terms of child protection in the USA, it's hard for us to give specific advice. But to be harsh - doing nothing, knowing what you know, is tantamount to colluding with the father in the abuse of the vulnerable young children.
Please, do something
What gives you a warm feeling? 🥰
Font size very small but only on this site