Your source (the Maseno staffer giving his impression of what is in the record to a reporter who is writing down the interview and subject to error) says “his elder brother, Joseph, left for unexplained reasons in 1951.”
Okanda is reported to be reading from a numbered index card for Sr. but NO card number is given nor is he reported to support his recollection of the older brother from a card. This does not remotely form the basis of a claim that Sr. had an older brother who is not recorded anywhere else.
The Yusuf in the family tree that you yourself posted is a son of Sarah and younger brother of Omar of the liquor store in MA who was born in 1944. Although positioned higher on your family tree, all of Sarah’s children were born later than BHO Sr. and Sr. has NO full brother and NO older half brother.
There may have been a cousin from the clan that he mistakenly referred to as a brother, for example.