Dreaming about my friend's brother, and his girlfriend? Am I jealous?

So I've had a big crush on my friends brother for a few years now. He's 5 years older than me, and has said I'm too young for him. I've met someone else now, and have been with him for over a year. I still melt with lust when ever I see my friends brother though. I don't want to stop going to her place though, because we love spending time at each other's houses. I try and ignore him, but recently he's got a new girlfriend. He never had a girlfriend before, and always slept around. After looking at facebook pictures of the both of them together, I dreamt about them! In the dream I was at his house sitting on the couch, when they both came in the door and started kissing on the other couch right in front of me. In the dream he was putting his hand up my skirt a he kissed his girlfriend. Somehow his girlfriend didn't notice though. Why would I dream something like that? Is it a sign of jealousy?