First, Auma, on the corroborated record, is Barry's half-sister and is the child that Kezia was pregnant with when BHO Sr. in corroborated to have left Kenya in 1959.
The conversation you refer to involving Auma and the estate of BHO Sr. is ONLY sourced to dialog in Dreams which was most likely invented by Ayers and/or Obama (Ayers is quoted as saying he “wrote the whole thing,” IIRC).
Anything in Dreams, IMO, cannot be used as a primary source and is highly unreliable and this “memory” is uncorroborated by any open access Kenyan government or tribal marriage records or access to any family witnesses, IIRC.
There is an interview with Kezia up thread that I'm referring to. My mistake in that I should have said both Kezia and Auma. Kezia, in the interview, says there was no civil ceremony, meaning it was basically a tribal jump the broom type thing where nothing was legally documented. If it's not documented, then the marriage would not be recognized by American courts so the supposed marriage between Ann and Sr. would be legal.