Definitely don't tell your boyfriend. I had a similar situation with my boyfriend of 2 years, his brother is a year younger than him and so about 6 months younger than me. We've always got on well anyway because I dated one of his friends and so knew him seperately from "boyfriend's brother", he was actually my friend. Then I started to find him really attractive and he's so funny etc, he does remind me of his brother. I really fancied him and the problem was that it was a little obvious that he fancied me back, there was a lot of flirting and eye contact etc. Sounds quite bad but I guess we were just being friendly.
It did pass though, and now it's just back to being friends. Just concentrate on him, push the brother out of your thoughts. Maybe go away for a bit and I bet that it's your boyfriend you will miss, not his brother.
It is harmless, just don't act on it!