I need help!

I have a family member who is in an abusive relationship. They've been together 5+ years now. It started as emotional and mental abuse and blackmailing, but recently it has turned physical. I also found out that he has hit my niece (3 yrs. Old). My family member is young and she isn't taking it seriously and was defending the abuser.

I don't know what to do about this whole situation. My family is stuck.

If you've been in this situation, what do you wish your family and friends did? What can I do to help her understand she needs to leave him?