ya brothers mate is usually not a good idea, or for the guys, their sisters friend..
my son is 21, he was interested in one of his mate's sisters.. it didn't last, and nor did his mate stick round for long because she started causing a bit of trouble between them. its risky, but if you are mature enough and he is too.. then talk to your brother first and ask if he would mind if you got to know his room mate better.. or perhaps ask what his room mate thinks of you.. your brother will think he's being clever by dropping you in it which can sometimes make the room mate think of you differently, but not always. sometimes the chances are it's already mutual, but the age difference, he might not think of you that way. you might seem more like a little sis to him.
follow your instinct but do soem homework first :) good luck