You did a good job explaining the difference between American and English systems over the accepted polygany of Kenya. Obama Sr’s first stepmother had no children which is a big no-no if you will. The second wife had two children with Obama Sr being one of those. Sarah, the third wife was a member of the family prior to Obama Sr’s birth and essentially raised him after his own mother deserted him. It goes into the concept of “sister wifes” as mentioned by the FLDS but Americans cannot relate to having multiple wifes, with many men thinking one mother in law is sometimes more than enough!(Sorry I tried to put a bit of levity into the explanation!)
Akumu Habiba (sometimes referenced as Habiba Akumu), Hussein Onyango's second wife, had three children: Sarah, b. abt. 1933; Barack, b. 1936; and Auma, b. abt. 1944.
The birth dates are based on the narrative related by Sarah, Hussein Onyango's third wife, in "Dreams," where Akumu is described as having run away with her youngest, Auma, in abt. 1945, "One night, when Sarah was twelve and Barack was nine."
Sarah, the eldest daughter of Akumu, and BO, Jr.'s full-blooded great-aunt, is portrayed in "Dreams" as a nasty, money-grubbing creature who is responsible for all the acrimony between the various wives/children of BO, Sr., through her claims that some of them are not BO, Sr.'s children.
Akumu is currently referred to as "now deceased" on African news sites -- with typically no explanation as to where or when, with at least one reference to her "second marriage" to someone with whom she relocated to what was then Tanganyika. I haven't found any mention of what became of BO Sr.'s younger sister, Auma, (not to be confused with Auma, half-sister of Obama, daughter of Keziah, b. abt. 1960, who refused to accompany BO, Jr. to his visit to Sarah, daughter of Akumu, described in the book, sending the now notorious illegal US immigrant, Aunt Zeituni, daughter of Sarah, Hussein Onyango's third wife, as interpreter instead.)