Due to the so called "Braganza curse", casted by a monk on king D.João IV, neither of the firstborn sons
of any Braganza monarch, up to D.Maria II succeeded to the throne.
The only exceptions were D.José I and D.Pedro IV who were succeeded by their firstborn child, who were daughters
and could escape the evil curse.
After D.Maria II, the dynasty, technically changed to Bragança-Saxe-Coburg-Gotha and that might have shuffled the curse
, so both D.Maria and D.Luis were succeeded by their firstborn sons.
But with D.Carlos, the monk's curse reappered
and it was the second son, D.Manuel, who succeeded his father.
In Portugal, the 2nd and 3rd sons of the 1st Bragança king succeeded to the throne,
D.João IV had 3 sons. D.Teodósio, the eldest, prince of Brazil, predeceased him due to tb.
D.João was succeeded by D.Afonso VI who was removed from the throne by his younger brother, the Regent and later D.Pedro II.
A different category:
King by marriage:
D.Pedro III, younger brother of King D.José I, became king by marrying his niece D.Maria I