A neighbouring ruler had three kingdoms, all under gavelkind succession and limited or no crown authority. Furthermore, he had two duchies, with one county in each. One of the duchies is within the de jure territory to his primary title (his capital), while the second duchy is outside the de jure territory of any of the three kingdoms (but not within the territory of any existing kingdom or empire).
The king had two sons and a couple of daughters. I would expect the two sons to get a duchy (and county) each, and the eldest son taking the primary title and one additional kingdom – thus splitting the realm.
However, when the former king finally kicked the bucket, the eldest son inherited all of his titles, while the younger son remained landless.
Why did this happen?
Actually, the game doesn't list the younger brother in the inheritance anywhere (except for a duchy outside the realm), even though I see no reason for him to be disqualified. He is not a bastard or priest or anything, but he is a brother from another mother (the old King remarried).
Best Answer
I think the reason is, that the old King had two counties, but one of them was occupied.One of the (African) dukes of Aquitaine is fighting a war of independence against Aquitaine, and he has occupied one of King's two counties (the one that is not the capital).
I investigated by loading in as the elder brother after succession, and see if something prevented the distribution of titles. Sure enough, the only title could grant my younger brother, was my own capital in Aquitaine, after which my capital was automatically changed to the occupied county in Africa.
I assume that the capital should go with the primary title. If the younger brother was to inherit the occupied county, he would be independent, and the duke's independence war would be invalid. Rather than ending the war, the game apparently chooses not to let him inherit.Alternative
The wiki also states, that in terms of gavelkind succession, the game tries not to split realms illogically and produce too much border gore. So a second explanation could be, that since the King held no de jure territory in his two "secondary" Kingdoms, the realm was kept together.
... however, I opened the game as the king, won the conflict and waited to die. This changed nothing. I even tried some variations, where I manually landed the younger brother up to duke level, and even tried changing my primary. This does not change anything.