Leave her. You shouldn't be dating anyone. Everyone else: If your SO insists that you not see your friends, that person is an abuser who is in the process of isolating you from all other sources of emotional support, and from anyone who might empower you to leave when things get really bad. It's a huge red flag. Get out now, while you can.

You need to get as far as you can from this one! Can you imagine going to a family reunion with this one?

People can, in fact, maintain friendships with people that they were once attracted to. Hooking up in the past doesn't mean it'll happen again- it could be that they both agree they're incompatible in the bedroom/as a couple or they may have concluded that they just don't feel that way about each other anymore. You need to grow up and stop trying to control your girlfriend, or else you'll never have one for long.

I'm friends with all my exes. And no man will ever tell me I can't be friends with them!

you would be out my door so quick your head would spin

You can absolutely have a purely platonic friendship with expartners. You can also tell your partner what they're allowed to do and what not. But you shouldn't because then you're an insecure brat that projects their fears onto others^^

I see where you’re coming from. There’s a difference between having friends and maintaining a friendship with someone you have had a sexual or intimate relationship with. If it’s not something you’re comfortable with and she won’t end it, then leave. Just please do so like an adult.

If it was a recent relationship that had been long standing, I could understand but if it was a one time hook up 6 to 8 months ago I would not be concerned. If it wasn't even really a long-term relationship and wasn't something they were doing continuously. I think your reaction is overboard. I think your reaction is overboard.

How normal does said exes think it is to hook up with your brother!?

If you trust her so little then it's probably already over.