I'm sleeping with my ex's best friend even though she told me not to.
Do you think it was fair to sleep with her?
I will tell you one thing that I think it's fair game. Her friend has always had a crush on me, since then my ex told me not to even talk to her.
Even if you are no longer together, they are still friends and friends should not betray each other like that. You could cost them their friendship.
I don't know, maybe I'll end up getting one of them.
You'll end up getting no one and you'll destroy their friendship. Your ex's friend should understand that she's not doing the right thing and you should not encourage her. Find someone else to date.