Are trust issues causing me to over-react?

I’ll try to keep this as clear as possible.
My brother, with whom I have a pretty poor relationship has moved back to our hometown. There’s a history of physical, emotional and sexual abuse. It’s gone unsaid for years. He’s tried it on with a couple of my friends, but hasn’t gotten anywhere.
Until tonight. He spent the night with my closest friend. She isn’t aware of everything, but knows that we have a difficult relationship. We’ve talked it over and it was very clear to me that she would not go there – she specifically stated this.
I feel somewhat betrayed that she went ahead and spent the night with him. I feel so angry with him for coming into my life and working his way into my closest relationships. I feel like I’ll never escape the creep.
I haven’t spoken to my friend about this yet, but I’m finding it hard to figure out if I’m over-reacting or if my feelings are justified.
Also, I can’t figure out why she’d state clearly that something like that would be far too close to home and then go ahead anyway.