<human>: Can you condense the text into three sentences or less?

My boyfriend has been going out to the pub more often with his friends, and a few weeks ago he went back to this girls house to continue drinking with his friend (we'll call him Steve). My boyfriend works with her, and her sister is in a relationship with Steve (they actually got together at this party.) So the four of them were at this girls house (we'll call her Jess).

Me, Steve, Steve's brother and my boyfriend were hanging out last night (New Years), then while we were alone Steve's brother tells me my boyfriend had sex with Jess. He was told this by Steve's girlfriend.

I trust my boyfriend a lot but I want to ask him about this, I always believed he'd tell me if something like this would happen since he's usually honest. 

The thing is I don't know this girl well at all, and I don't want to sound like I'm accusing him. I think he would get upset if he wasn't actually cheating on me.  I go to college with Steve's brother and he's honest so I don't think he is lying about being told this. He mentioned that he brought it up with Steve (who was there that night) and he denied it, but he's not so honest and may be lying.

So I had doubts about him and thought he might have slept with her after the party, but tried to brush it off, but now I've been told he has... from an unreliable source through a friend. It seems ridiculous to say "Steve's girlfriend told Steve's brother who told me you slept with Jess" since, rumors, you know? How do I ask him about this? I've never had to deal with a situation like it. Also, should I ask Steve about it?
<bot>: A girl from where my boyfriend works may have slept with him at her party. Her sister told my friend it happened. My boyfriends friend says it didn't. How do I ask him?