I have a crush…but I'm socially awkward?

So… There's this one girl (let's call her Stacy) and she's really pretty. I've known her since kindergarten and don't really remember talking to her then. Now that were in high school, she rarely talks to me. But I can't help but feel she might have a crush on me too. She seems to laugh whenever I make (terrible jokes to her and my confidence boosts a bit because of it. But then it quickly goes away soon after.
So most recently, during this school year, she's been passing by my bus when school is over because her's is directly behind mine. And I sit in the second seat, so I can see almost everybody walk on their buses. But when she passes by my window, I can't help but feel like she subtle glances at me, almost in a nervous way. When we (accidentally) make eye contact, she immediately looks away… As do I.
So I'm really hoping that she might have a crush on me too. But I'm just worried that this might all be in my head and I'm just imagining all of it (I have a tendency to do such things.) But this time feels more real than others. There's just one other big problem.
Her brother… Is one of my best friends. So let's say I do ask her out. He could either kill me or maybe even accept it. I'm not sure which.
Anyway… I would really like some answers from people more experienced in the dating area or least can relate to me. It would help a lot. Because I might consider asking this girl to the prom if I get enough confidence. *bows* Have a nice day!
(*You-sleeping*)
Added (1). Wow. I just realized how long this. Please read the whole thing. I would be most grateful. I usually don't care about this type of stuff, but this girl seems really nice, smart, and pretty. I just hope I'm not diluting myself.

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