Love this girl, she likes someone else...should i..?

I'm new at this school this school year. theres this girl who rides the same bus, she is one grade ahead of me, but i got held back a year sho she is only 2 weeks older. we r both 17 anyways, we've both never really had a 'real' relationship, me because i'm just shy i guess, her idk why. anyways, when i first met her i thought she was kinda cute, but once i got to know her i basically fell for the girl. anyways i'm friends with her brother kinda, and he knows i like her, thanks to a text that was ment for someone else, and he warned me that she likes someone else, but he won't tell me who. i really like this girl, and i talk to her almost every time i see her, which is strange to me cause usually i'm shy and quiet. i was going to aks her out this week but her brother told me she likes another guy, now i dnt know what to do. should i ask her out anyways and hope for the best or what? she dosn't have a facebook, so i can only talk to her on the bus. again, should i just ask her out or any suggestions?