Im in a terrible Predicament and need some quick guidance?

[Drake Remoray]

Okay So My sisters boyfriend lives with me, he has his buddies over all the time to hang out, drink beer and watch hockey. Hes been dating my sister for 2 to 3 years and is usually a good guy. Last week though my sister was upset with him due to a conversation he had with his best frien, she overheard about a girl that his friend likes and use to date herboyfriend. He was insisting on inviting her over. Though we were partying and everyone was a few beers in I believed that it was an innocent gesture for his friend and swayed my sister from being upset.

It worked she was not upset anymore, it appeared as if we were all back to being happy and then 5 days later.. Im hanging out with his 2 friends and him and the other guy blurts out "So you know Birdy ( he described an appeareance ive never met any of the girl/girls in this) wants to have sex with you. And my sisters boy friend responds Oh yeah... thats nice"

Although i am not sure if its the same girl Im still in a predicament. Because if i tell my sister she may be crushed and that will open a whole new ball game on the consequences of all our actions. And the fact i stuck up for him 5 days prior dismissing these claims as though they had no merit and now with the new news ...... i dont know i really need some help here

Thanks a ton