Why stop at the brothers or even the parents? Why not demand that the entire mishpacha, aunts, uncle, cousins, grandparents on both sidea be available for a quick study of their middos, too?
Should a girl whose brother(s) are not up to your standards, not be good enough for you even if she herself is a prize jewel in every way? it’s time to stop over-analyzing everything, and start evaluating a person based on his/her own merits.