Yep, I agree with Masinor. Is it actually the brother that you like or is just that you are unhappy with your current boyfriend and are being tempted elsewhere?
This could potentially be a very damaging situation for all involved, I would make sure that I was 100% sure what I felt and wanted before I did anything at all or said anything to anyone.
Maybe take some time out from the shared house, your boyfriend and his brother. If you are away from them both and have time to think, it might make things clearer?
Good luck, I can't begin to imagine how hard this must be.