I am so sorry. What a horrible situation. I would be very honest with her, and let her know that violence is unacceptable from yourself or anyone else. That it WILL NOT happen again, and don't let it. Know when to walk away for a while.
It does sound like counseling would be a benefit for both of you. Maybe give her some space, living with the elderly is no picnic and she may not want that long term. Let her know you support her and that you will be there for her.
I heard my DD who is 14 tell her friend the other day that no matter how awful it would ever be, she knows I am there for her no matter what would happen in her life and that she loves me for that so very much. It brought me to tears because it is so very true. I will love her, cheer her on, all of those things. However, when push comes to shove, I won't be manipulated or if ever in her life she is a user I won't enable her in any way. I will have boundaries.
One thing to remember is that kids go to who is safe. So many times they take anger out on their own parents because they know the parents are constant, that even after it's really bad, we will still love them.
Be patient. Work on you. Try to have as open of communication as possible. Make amends. That is all you can do, and pray.
Tight Hugs. We will walk this with you.