My sister hates me and i don't how we could be the same anymore?

My sister hates me because her boyfriend came onto me while i was babysitting my 1year old nephew.. He was asleep and didnt see nothing as it all happen ds away from him... But i told my bestfriend what had happend that night because i was scared my sisters bf would have told her anyway during a fight as they do not get along well. Anyway so my bestfriend told my sister as he thought it was the right thing to do... And you can imagine she did'nt take it very well and i don't blame her... But shes more mad at me than her boyfriend... And i even herd that she belives his story over mine which was "i made the move on him" when i totally didnt... He wouldnt leave me alone when i tried to stop this... Yet she hates me more than him... And shes forgiven him and letting him ba k in her life again im so angry because he doesnt deserve that 2nd chance :/ but shes willing to give it to him? After hes just basically cheated on her... Why would you do that? He has abit of a reputation and she knows he lies to get himself out of things but shes chosen his word over mine... Now its gone to the point where we have to be in seperate rooms whenever shes round to visit because she can't stand me and i guess i cant stand being in a room with her now after shes belived that son of a b*tch. I want to be close to her afain but i don't think thats gunna happen anytime soon.... Do you? So help me out here guys... What the hell am i suppose to do :/ im also not aloud to see my nephew anymore either.... Im shattered.