Is it wrong to date your best friends brother?
No, it is not a bad idea for you to date your best friends brother-in-law. Both of you are adults and should be able to have a relationship based on how you feel about one another instead of how other people may feel.
How do I tell my best friend I like my brother?
Tell him you think youre starting to grow some feelings for this guy, but you dont want to hurt anyone or make the situation uncomfortable. See how he reacts and just talk openly and honestly with him. You dont need to know exactly how you feel or what you want, just tell him whats going on in your head.