Girls and going out..advice?

ok heres the story...i have a friend that i know now for about a year and a half....i really like her and shes going out with a guy dat all he does is juzt smoke everyday. dey have been going out for about 3 months..i hang out with her alot more then her bf does...im really cool with her and i've made her laugh many times..her sis tells me dat she talks **** about her own bf, and dat she doesnt want to go out with him..but everytime i ask her "do u relly like him" or things like that she says yea..she went to prom with one of my good friends..after prom she juz left my friend hangin to go out with her bf now...when shes with her bf she trys to talk to me and say hi and i juzt say hi bak..i really like her and i wana know wat i can do to be bf gf with her... please give me sum good advice...i dont care how long it is, just help