And Abijam slept with his fathers; and they buried him in the city of
David: and Asa his son reigned in his stead. 1 Kings 15:8
Abijam was Asa's brother. (They both had the same mother, Maachah.)
Now in the eighteenth year of king Jeroboam the son of Nebat reigned Abijam over Judah.
Three years reigned he in Jerusalem. And his mother's name was Maachah, the
daughter of Abishalom. 1 Kings 15:1-2
And in the twentieth year of Jeroboam king of Israel reigned Asa over
Judah. And forty and one years reigned he in Jerusalem. And his mother's name was
Maachah, the daughter of Abishalom. 1 Kings 15:9-10
The Harper Collins Study Bible note for 1 Kings 15.10: "Acording to this verse Asa and Abijam both had the same mother (cf. v. 2). This would make them brothers rather than father
and son (v. 8) Alternatively, it may be that Maacah held the important post of queen mother but was not necessarily Asa's biological mother."
It is possible, of course, that Abijam married his own mother (Maachah) and was, therefore, Asa's
father and brother.