Aggression in female lab toward male brother (neutered)

Question
We got our well-bread English female yellow lab at 3 months from a
reputable breeder, and she has a stubborn streak in her, and developed the
quirky habit of staying under our bed much of the day ever since our
grandchild came to live with us for a few months (not a permanent situation).  
A couple of days ago, in an effort to bring her out from "living" under our
bed, we decided to try having her brother (litter mate), both 13 mos. old)
come live with us, but they have always played very roughly together, from
the beginning (which concerned the breeder with whom he lived).  We had an
episode today where our female just went "off" on the sweet male quite
viciously &aggressively (& seemingly unprovoked) and he naturally had to
defend himself.  They were outside when this occurred. I thought they were
going to kill each other!  I was shaken by this behavior on the part of our
female -- IS THIS A BEHAVIOR THAT CAN BE CORRECTED IN HER, AS WELL AS
TEACH BOTH DOGS TO PLAY MORE GENTLY TOGETHER, or is it just a bad idea
to bring the brother into our family?  He is much sweeter than she is, and
we've fallen in love with him, but she was here first and has been our "baby".  
They are fine around each other inside the house.  Please answer ASAP!  
Thanks.

Answer
It's hard to gauge her behavior without being there with her.  I would address this situation with the breeder and have them spend some time with her to see what's up.  Perhaps SHE needs to go to the breeders house for a few weeks and see if the behavior continues there, or if it's only something that goes on at your house.  

Females can be a bit more domineering than males, and perhaps she just doesn't like the other dog in her territory.

Have you had her out alot for socialization and training?  If not, you might want to enroll in a class.