Which father's half sister would this be? The father he grew up with, or the father he hasn't seen since he was a toddler? I might be rather off on my Obama Biography details here, but I think there's a "missing" father in his family somewhere.
FYO I have a father I haven't seen since I was 3, and 7 half-brothers and half-sisters that I have never met, and never intend on meeting, due to messy, tragic circumstances and moral and legal problems. I don't feel morally obliged to support anyone on my blood-father's side of my "family".
Neither do I feel ANY moral or emotional obligation to my mother's parents, who are the most actively destructive, evil and malicious people I know who act within the law. I would not give one sou to them, and I do not expect, nor do I wish, for one sou from them.
On the other hand I would sell all my belongings and live in starvation with anyone on my extended step-family's side, if it was necessary. Fortunately my personality is compatible with living with my step relatives.
My mother, sister and half-sister I love to death and would die for. I'd starve for my mother and sister, but I wouldn't and couldn't live in the same house with either of them. We'd kill each other within a fortnight, since they are more than highly strung and in dire need of large personal space. Also, they have convulsions if anyone touches their stuff. ANY of their stuff.