Help! Relationship problem..?

Im talking to this girl. I keep hearing things about how shes talking to other guys. I confront her about it but she denys it. The thing is my best friend is talking to this girl's sister. The girls sister told my friend all of the guys shes playing. theres 4 others not including me. what can i say to the girl im tlaking to? shes going to deny everything that i say. i cant let on that my friend told me about the 4 other guys or else everything will be messed up with him and my girl's sister. please. what do i say to allyson (the girl im talking to)