This always stood out to me in Priest's Black Panther run but it stands out more so now that T'Challa has a new half-sister Shuri.
T'Chaka had a unnamed wife from a tribe that wasn't Wakandan that gave birth to his son Jakarra. Now since T'Challa was T'Chaka's heir we can assume T'Challa is the eldest.
Modern comics mention T'Chaka's first wife N'Yami who is T'Challa's mother and they mention Shuri's mother Ramonde. But no mention is ever made of the other wife or Jakarra. Presumably Jakarra's mother was the 2nd of the 3 wives yet modern stories don't seem to support that idea.
Has T'Chaka's second wife and son been forgotten?
Yep. Along with his other "brother" The White Wolf, whom I really liked.