My sister and her 3 kids are being abused?

My 29 year old sister is in a very abusive relationship and her 3 kids are scared of there father.I am trying to help my sister called me last night crying about a big 3 day fight her and her boyfriend had over the weekend.I had there 3 kids for the whole weekend and the kids were begging me to not take them home.It killed me that I had to take them home because my sister called and told me there father wanted them home or would call the police.I have called florida dcf on him already and they didn't do anything because he refused the drug test and got a lawyer and got out of it.He did take away her cell phone last night like always and I live 2 1/2 hours away.I want to do something but don't know what I can do with out being put in jail.I have a 5 bedroom house I have plenty of room for the kids and my sister she said she was going to tell him last night that she was leaving him and that is the last time I talked to her. I may drive down there or call the police to do a well check but I don't want to over step my sister but I also want to protect the kids.Please any advice would be great.I can't stop crying.