What's up guys,

So in my math class there's a girl who I had sat next to for awhile and over time she became "crazy" for me always touching and starting conversations and getting close, wanting to know more. Long story short she is attracted. We switched seats so I told her we should hang out outside of class and we did today, went awesome.

I let my brother know what went on today, he proceeds to DM her on Instagram and try to talk to her. She texts me and tells me what he did. I flip on him because I myself moved the both of us out of our crazy moms house and I've done everything for him since, and he has the balls to do this. Btw he had done the same to our stepbrothers girlfriend back in high school and took her from him, that was his first love. His intentions are clear here.

Now she's asking why he texted her and I don't know if I overreacted by flipping on him. What do I do in these kinds of situations?