Could just be bruising, or light trauma. Usually when they're brain damaged, you'll know it, and it usually doesn't get any better. Or they just act "funny" later in life.
Of course, i've never seen a chicken with brain damage, but other animals. I dropped a baby Budgie about 4 ft onto the floor when I was handfeeding, he had blue marks around his head, but quickly recovered.
I am also a groomer, and had a dog spaz out once, jump out of my arms (just came from tub), land on the floor, and walk sideways for about 20 minutes, and then was perfectly fine.
So it could be minor, or temporary...only time will tell.
As for your little brother, don't be so harsh...kids are kids, they don't always think before they act. I'm sure your little brother has a good heart....he just let his temper get the best of him. Teach him how to properly care for the chicks, he may enjoy it just as much as you do
I have a 3 yr old boy and an 8yr old boy, so I know it all too well. They love their birds, but at this age, they cannot be trusted 100%.