Maybe ask ur brother if he still likes her and just tell him that you are want to ask her out and see how he reacts. If he didnt asked her out yet then i would say you should give it a try, you cant keep waiting for your brother.

Yah I've been in a similar situation. I used to date this one guy and his best friend had a crush on me. Me and the guy broke up and a week later the best friend asked me out. They were awkward for a while, but whatever happens, it was really between those two duking it out. Same thing goes for your ex's friend. She's gonna have to duke it out with your ex if things get awkward, but it'll all work out. In the mean time, definitely give your bro a heads up.

Beware - if the girl you like is a loyal friend, she might say no for the sake of your ex, I've rejected a guy on the basis my best girl friend had been out with him and still liked him. Also she might reject you if she thinks she's just your rebound to make your ex jealous. I'd give it some time if I were you. Good luck

I would tell your brother straight up about it, I was in the situation but as the girl... I liked one guy who was my friends x & his brother was in love with me... Me&the one i liked dated for awhile secretly but when his brother found out he tripped & now they don't have the same brotherly bond... & they lost it for a relationship that didn't even last...
So just be upfront & tell him that you like her & are going to pursue her...
& I agree you should wait a few more weeks before you make a do you want to be my girlfriend question... let things cool down, & make it seem like you needed time to move on.