Hello!
I am sorry this has happened.
Some affects can be major and some can be minor. This depends on the speed in which the car was traveling, whether or not that person was wearing a seat belt and what type of car it was.
Worst-case scenario: Your brother can have bleeding in his brain due from hitting his head hard, you can also have a coma and also a major concussion
Best-case scenario: Your brother walks out in a few hours with just a few minor scratches and bruises and maybe a minor concussion.
I hope it helps!