@burcidi-- I'm not an expert on this. But I think that most of the time the fluid is absorbed by the brain and disappears on its own.
The major thing to worry about with a basilar fracture and leaking of CFS is meningitis. And also if there are hearing problems, or loss of other senses like taste, there might be something more serious going on for which the doctors will have to intervene.
But if they sent your nephew home, I'm sure his situation is not serious. Your sister should keep a close eye on the CFS and also check for meningitis symptoms. If he does develop meningitis, it is easily treatable with antibiotics as long as it is noticed early.