Why is his brother so cold to me?

Ok , here's the thing, it cold be many things. but most likely I think you might be taking it the wrong way. More then likely, and I'm not saying this to be rude. But "YOU" do not matter to his brother. what I mean by that is he doesn't LIKE LIKE you ,or care whether or not yall are together.He most likely doing it towards his brother and you take it more seriously. But it can depend on how is brother is . If he is a y guy, its normal, if he's a quite guy , its normal. if he's shy, then maybe he might like you. but heres the thing. that also refers to you boyfriend on how he acts and treats his brother. well, sorry couldn't help much.good luck.