Why are brothers so protective of their sisters?

Anonymous
My brother and I don't speak, we've never been close and he knows I don't care for the whole alpha male "you can't date this person because I say so" bullshit. yet when it comes to his friends or guys he knows he literally threatens them to stay away from me. What is that all about? I don't understand it.
Why are brothers so protective of their sisters?
8 Opinion