In any case, why the heck didn't the Obamas just sponsor the aunt?
From this link, FWIW....
Is an immigration sponsor responsible for immigrant beneficiary receiving federal assistance
The statute regarding the Affidavit of Support, Form I-864, states clearly that in order for an alien to immigrate through a family-based petition, and some employment-based petitions, they must overcome the public charge ground of inadmissibility. Therefore, the petitioner on the Form I-130, Petition for Alien Relative, must execute and submit a legally enforceable Affidavit of Support on Form I-864. The second paragraph of the instructions to Form I-864, advises the sponsor that by executing the form he or she is agreeing "to support the intending immigrant and any spouse and/or children immigrating with him or her" until the contract terminates. In cases where the Affidavit is executed by a spouse, even divorce does not terminate the obligation." Part 7 of Form I-864 states: "The sponsor must provide the sponsored immigrant(s) whatever support is necessary to maintain them at an income that is at least 125 percent of the Federal poverty income guidelines."
One of the things about this story that really bugs me is that there seems to be no teeth in decisions made by the Immigration Department. Who is responsible for enforcing these decisions?