The teacher at the Maseno school said that the older brother, Joseph, (english for YUSUF) left the school 'for unexplained reasons' in 1951. I gather Yusuf was two years older.
For some strange reason, wiki has a birth year for Yusuf as circa 1950's. Here's the family tree, showing Yusuf:
There is also another half-brother, Omar, who shows up in Boston, under the name OBAMA ONYANGO:
Obviously, YUSUF, OBAMA AND OMAR are of the same generation...so why are they hiding Yusuf?
This has to be Yusuf. The eldest son of ONYNAGO and SARAH. He would be the Clan Elder, that's why he wears the cap and carries the whisk, symbols of his position.