It doesn't mean anything, a dream is a dream. It's not always going to come true - maybe you were just thinking about him before you went to bed and had watched something about brothers and sisters sleeping together and it all just got confused and put into your dream. Sure there is nothing to worry about x
I think you love your boyfriend so much that you feel some type of insecurity about anyone who cares for him... This type of dream only happens when you feel insecure, try to forget it and make yourself full of positive thoughts, and take care!
It's just a dream - quit thinking about it and try to get it out of your mind , in this case only you can help you.
It's just a dream! Means nothing! Tell your boyfriend about it, I'm sure he will think it's funny and nasty.
Just a very abnormal and strange mind you got there! All you can do is forget about it. Don't worry, it doesn't mean anything serious...just yet!