Hi there. I'm a youth worker who works with people aged 14 - 25 years of age in England. I work mostly with people outside of society, or in danger of finding themselves there. I come across domestic violence from a couple of angles, both the person/s being abused directly and those who witness it.
I go with Joe and the rest. Contact the experts. There is something else going on within this process though, and that is you, how you feel and how you deal with those feelings. Well played on being willing to discuss it. I would encourage you to recognise that you are hurt by this situation too. You have a right to some healing and some help.
Good luck with it. Often there are no winners. Often nothing we do works, for a variety of reasons. Sometimes you plant a little acorn that doesn't look like it's done anything, but later on (perhaps months, perhaps years) it turns out to be a big sod off tree that no one knew was there and that proves bloody useful!