What to do and where to start?

So i like my sisters friend who i have known for a couple years now back when we first met she liked me but it was a baby crush since she was 13 and i was 15. Okay she doesn't hate me now but we really don't talk a lot and have never hung out but i know a lot about her from my sister and just seeing how she is with her friends etc i Like her since shes not afraid to say what she believes in and isn't afraid to tell you what she thinks of you. She defended me not to long ago by cursing out my ex because my ex seriously played me by going out with me for 4 month's then breaking up and hooking up with another guy a week later but she said i was like a big brother to her while she was defending me. I see her every now and then and we talked a couple times on facebook the thing is i feel like i dont know what to talk about with her and i dont want her to think im boring when i know im not lolz its just i get so shy over what to say that i end up not saying anything at all and its really frustrating also since i dont even know if she even thinks of me as anything else but a brotherly figure i know shes looking for a boyfriend now and i want to be her boyfriend but i just don't know how to start if i can't even talk to her usually im cool and confident and know exactly what to say but she just really breaks me, for example yesterday i saw her at work and she hugged me but while we were talking i ended up not knowing what to say and it ended up sort of awkward on my part. so advice? do i have a chance or will i forever be the brother figure? and if i do how can i show im interested and get over the shyness?