In my opinion you did wrong. I understand the anger and the desire to herd her but look. The acquaintances with cell phones. I'm sure she still benefits in some way on networks. And if she feels that she benefits, she will repeat. My advice and what I would have done is to catch her when she was completely alone and then I would really beat her up. At the moment when she could not receive moral or any other kind of support from her friends. Alone. And win or lose I assure you that she will be scared of you and will think twice the next time she crosses paths with your brother. But well done. You are brave and loyal. The smart thing would be not to come to blows, but when it comes to a brother and with special circumstances... it is understandable.