The reason why the Torah says that you should check into a girls brothers before completing a Shidduch, is that being that they were raised in the same home chances are that they will have similar behavior(ism). However in todays world we are influenced by the outside world and we can’t judge a girl from what you see in her brothers…
This is the Daas Torah from a Rav that answers questions for http://www.shidduchworld.org