Supporting a son or son-in-law in learning – is it a good investment?

Is there an obligation to support a son (or son-in-law) in learning?
If the father (or father-in-law) commits to support, can he get out of it, for example when business is bad?
Does living comfortably facilitate or get in the way of quality learning?
Is it best to fund the strongest learner, the neediest person, or a relative?