What to do if adult child out of house knows of abuse in parents home?

ok so my question is confusing but the situation is complicated. Both of my parents drink to much it has been progressing over the years and i feel is a time bomb. I have a brother that is a recent veteran and sister in law living with them along with my younger sister. My brother and wife are like myself and want to help but know you can't change them . My true concern lies with my sister. She has a lot of mental and emotional issues causing her to act out including having at a young age. Things get violent nightly i would say maybe well it used to be 2 times a week maybe but from the sounds of it now it is every night. I moved 2.5 hours away and don't talk much to them after many failed attempts of trying to get them to get help. I don't want to give up and have something bad happen and know i didn't do enough put I don't want to report them and have them loose everything either. All i want is for them to seek the help they need and help my sister , grow up and be parent