What do i do about this?

For a start, you need to break up with your boyfriend. He'll be hurt because he loves you, but it's much better than leading him on, pretending you like him when you're actually in love with his brother. How does his brother feel about you? Once you've broken up with your boyfriend, then you are free to see if you have a chance with his brother, but consider your boyfriend's feelings too - how will he feel if you start going out with his brother? Be careful, because there's a chance that a lot of people could get hurt in this situation.