OK, I just spent an hour looking for this fic and I cannot find it. I'm pretty sure I know the one you're talking about though, so I thought I'd add a bit to see if it helps jog someone else's memory on the title:
Don was driving a witness somewhere and Charlie was with them--why and where I can't remember. They're in a car crash--with people who are after the witness. Don saw it coming but couldn't avoid it, instead spinning the car to make sure they're hit on his side--protecting Charlie, but forgetting until the last second that the witness was on his side. Charlie is still conscious and tries to defend his brother, and gets badly beaten as a result. Don wakes up in the basement and wakes Charlie a few times, though the witness never wakes. He keeps getting Charlie to drink water. They were left in the basement to die, and the witness did die. The smell from the body got a nearby neighbor's attention, and Don and Charlie are rescued. Charlie's doctor told Don he probably saved his brother's life by making him drink water...
And that's all I can remember. Anybody know the title?
If I'm wrong and this isn't the right one, sorry. :-(