Depends who you work with and your relationship in general, personality, their position in the company etc... I've worked with my:
Dad, all good,
Brother, that was all okay
Sister, that was terrible
Nephew (oldest), that was terrible
Nephew (youngest), I got to help me on odd jobs as he is willing to learn but I just didn't have patience for him
Its a gamble, depends how your brother runs the business and what your relationship is like. He may be pushy, he might treat you as an equal and more of a partnership. Its your call at the end of the day as he is your brother. There's always pro's and con's and possibly personality/decision clashes.