How to avoid a start-up guy?
uptown: If I were you, I'd just tell him it's not something I want to work on, and leave it at that.  If you need an excuse, tell him you don't want to mix business ventures with personal friendships ... even though it's your brother's friend, and not yours.  Based on your post, it appears that the relationship is already being strained because of the project, and it will only get worse if you were to assume some level of responsibility.Your last sentence mentioned that the startup guy is excited about a particular sector.  Are you excited by that sector?  If not, then it's another reason not to get involved.