K.H.
I am that girl! Kid 3 and the only girl. There have been times when I've wished for a sister, but I'm happy with my brothers. I get to be the favorite daughter no matter how badly I screw up! :) I married in to a family of all boys so my SIL and I are pretty close even though she grew up with all sisters. I'm also friends with my brothers' wives (one only child and one with a brother).
I don't think having all brothers is going to be a detrement to her unless you tell her it is. Don't worry, Mama, she'll be fine.