Best Friends Brother.. Oo Er

Well when i first heard about my best friend's brother, i thought he was pretty cool, n i met him and he was

we have become quite good mates, which my best friend doesnt like as they don't get along very well... we are both quite sexually attracted to each other, and have told eachother quite frequently..

but recently i have started to dream about him, kissing, having sex or generally being there

I have a boyfriend who i am totally in love with, my fiance', but i cant help thinking about him aswell sometimes...