Generally, a 55cm will fit someone with your brother's specs unless he has unusually long arms. I'm 5'11", regular build, 33" floor-to-pubic-bone inseam (I buy 32" inseam pants) and 55cm works well. Fit can be fine tuned by choice of stem length and seatpost setback. A 54cm or 56cm are close enough to work as well.