Ask QC: Do I Tell Her?

Dear QC,
I’m not sure how to deal with a recent situation. I live with my boyfriend, we are in an open relationship sexually, no problems there.
But last week, my sisters husband came over to return some DVD’s and as both our partners were working late that night we were both at a bit of a loose end, nothing much to do. After a few drinks I was quite shocked when he asked what’s it like to fuck a guy, then asked if he could try it with me. He even started to try and touch me and I could clearly see his erection through his jeans!
Although I laughed it off at the time, I put it down to the drink and thought what the heck he was probably kidding. But he’s since hinted at it a few more times and recently started flirting with me and touching me up when no one is watching. What bothers me is that not only am I sexually attracted to him and being tempted, but I’m worried that if he is flirting with me like this, then he’s probably doing so with other guys and ultimately going to be unfaithful to my sister.
As far as I know she has no idea he is bisexual and obviously she has no idea he is trying to get into my pants. Ironically, she even says how great it is that he can have gay friends like me, but I’ve started making excuse to avoid him now. Do you think I tell her what he’s up to and if so how’s the best way to go about this? Do I tell her everything? I don’t want to hurt her but I also don’t want to see her get hurt by him either.
Thanks for any help.
J.