I’d never put my hands on a girl let alone my sister. He should’ve just cut ties with her and continue with his life.
I see where you're coming from but young girls, like 12-16 will do the dumbest shit and regret it when they're adults. I think if a girl like that has an adult sibling, it's their responsibility to intervene before cutting ties. Of course, the maximum for me would be light physical punishment (no more than a slap on the forearm. Just hard enough for her to go 'ow' and no more). If that doesn't work, then the only thing you can do for her (or him) is to pray and let them be.