Goddammit my brother, who is also my appointed heir and designated regent, is a retard.
My current Aquitaine game has conquered maybe a third of the Iberian peninsula through long, hard work. As luck would have it, the Pope just called a Crusade for Andalusia which would be a wonderful addition to the kingdom but at the same time, my king got his skull bashed in during a battle so he's incapable now. And despite the Ummayad's being weakened due to a couple of internal wars and uprisings, my brother apparently doesn't want to be the owner of both the kingdoms of Aquitaine and Andalucia. I guess I can just hope that my ruler dies before it's too late to join the Crusade.
Edit: That was a tense couple of years. Lost of couple of counties to regent shenanigans and the Crusade warscore reached 93% before the Ummayads managed to turn it around. Luckily, my incapable ruler had the good sense to die just after I appointed his son his heir instead of the useless brother who was apparently set on turning over Andalusia to our main christian rivals on the peninsula, the Italians. As soon as he was crowned the son joined the Crusade, which was now down to 17% warscore, turned it around almost single-handedly and managed to win the rest of the Iberian peninsula, save a few stragglers and Italian provinces, in a decisive war that made him known as "the Great" at 20 years of age, in a span of 2 years.
I love this game