Hey Josh,
Hope the little bro pulls through, and things turn out better than the information you've been given. There are stories of triumph all the time.
As far as his brain swelling, it's a cycling effect. High ICP (intracranial pressure) can cause additional injury, and further swelling.
The brain is much like any other tissue on the body. When it's injured, it swells. The difference is it has no place to go because it's contained within a solid shell (skull). An ICP of 24mm is high. Too high. An ICP of 30mm is considered severe brain swelling. The doctors are set to begin relieving pressure at 24mm to attempt to control further damage, and resultant swelling. There is a finite amount of liquid to drain off in the brain, and they don't want to begin relieving if they don't have to. It's not just the ICP they are worried about. The CPP is also very important. It's basically MAP - ICP. It has to do with the (mean aterial pressure) blood pressure, minus the intracranial pressure. If the ICP is elevated, the blood has a hard time being pumped to the brain, and the brain becomes oxygen starved.
I hope a little partial explaination will help you.
http://www.nda.ox.ac.uk/wfsa/html/u08/u08_013.htm
Good luck to your brother, you, and your family.
Kell