I was playing as the king of Al-andalus and managed to end the struggle for Iberia in peace, but I still wanted to create the empire of Hispania. I couldn't conquer Castille because we were allied, so I married my heir to their's thinking in 2 generation, I'd inherit both titles. What happened is that she became queen of Castille at a young age and somehow my son moved to her court and instantly converted to catholicism. Right after that, I saw the game indicated my player heir was now his younger brother somehow. I don't know if being a king and caliph changes something about inheritance? Is this an exploit to disinherit people for free?