WHAT TO DO???

The ex should be a closed door. If you broke up once, what's going to stop it happening again?

His brother... that's a naughty situation... well naughty. if you're dead set on his brother then talk to your ex before you do anything. If he is cool about it, sure.

The last thing you should do is cause family tension, because a/ it's just not right and b/you're going to be slap bang in the middle of it.

End of the day the choice is yours and yours alone.

B