What are the rights for children who live in a home where domestic violence is witnessed?

I called DCFS and was told that if the child is not being physically hurt then they can not help. The child does see my mother getting beat and also threatened. I feel she is in danger around her father. I do not want my 3 year old sister to grow up with unstable parents who are constantly fighting and abusing each other. I am 20 and independent what can i do if i am her sister? How do i go about getting custody of her? Please advise in detail - Is this your question? Add additional information