Best friend's brother

I have feelings for my bestie ' s big brother .. he is amazing .. We have so much in common...and when am around him I'm so happy .. 2 days back.. We were alone at her home with her Bro... she had gone out to the store to get something.. and her brother.. asked me out..even he is not sure how she would react .. but he wants to be with me.. he wants to take me out for dinner.. but am scared to tell her..what if it changes everything between me and her.. what is she doesn't approve.. but I really wanna be with him :(