I want to get with my friend's brother, but I think he's totally straight, not sure.?

You could always invite him to hang out with you and your friend and just include him more. If you are comfortable, you could also tell your friend what is going on and see if he can help you out. Overall, just keep talking to him and hanging out with him whenever possible. Avoid sneaking around and trying to only talk to him alone, as that may be suspicious or upset your friend if anything does happen. After you get to know him a little better, you can either come out to him or bring up a LGB topic, such as marriage rights or even a celebrity who has recently or surprisingly come out, then see where things go. Good luck!