they are not brothers they are friends and what are you like a homophobe or something and only the writers will know if they are or are not and it doesnt hurt you any if people wanna let their minds wonder and make them have a thing dont like it dont look at it
also it never said they are actualy friends eather The anime never even introduces Matt, But in the manga they are shown working together and also growing up at wammys. It does not show them as friends just acomplices. The only that shows they ever liked eachother is when Mello apologizes for getting matt killed. But other then that we don't really know.