Are you asking if favoring him makes sense or is justifiable? You feel good about helping him out financially & resentful about helping out his 21 year old sister who hasn't felt as close? Are you wondering if an obvious discrepancy between your "policies" about money with the two is something you can justify because of their relationships with you?
Well, it's something you can rationalize. It's a common inclination (to reward one & punish another, or give to one you're happy with & withhold from another if you feel disapproving or unhappy.)
Your feelings are your feelings & you can make any decision you wish to in response to them!
My thought would be to own your decisions, rather than to put it on your SD. Just own that because she isn't as close, and seems only to reach out when she is hoping to get something from her dad, you don't want to give to her. And that you want to give to your SS because you think he deserves it.
at 6:22 AM on Nov. 13, 2013