I am really attracted to a 12 year old, and im 19. What do i do?

I am 19 and for a few months i have been falling for the younger sister of a girl i went to school with. Im not like into that kind of thing it is just this girl. She is adorable, mature, and very funny. I have been afraid to tell her how i feel because i dont know if she or anyone else will understand. I dont want to do something that is bad for her which is why i am so against this. But I keep thinking about her. I dont know what to do and her best friend wants us to go out. I dont condone this from anyone but I really do care about her. What should I do?