I'm about to sleep with my brothers ex... please stop me.
Why can't you just stop yourself to sleep with him?
I can't avoid him because he's also my friend
Wanting to stop is proof that you can do it, but you should do it yourself and not depend on others help for that. Sleeping with your brother's ex will put a huge strain on you and your brother's relationship. One fling isn't worth it, just lock yourself in your house for the time being.
I just want to get in his pants.
Have you considered talking with your brother about it? He will find out anyways and as a last ditch effort he might be able to talk you out of it.