There is an interesting dimension to the fact that all kings' sons might be kings themselves. Maybe a little off-topic here, but perhaps worth mentioning in case anyone is interested.
In the epic poem Nibelungenlied there are three brothers Gunther, Gernot and Giselher, sons of Gebicca. They correspond to the historical figures Gundaharius, Gundomar, and Gislaharius.
They are all Burgundian kings. As the oldest brother, Gunther is the senior king and the only one called King of the Burgundians. His younger brothers are just King Gernot and King Giselher.
In the story, it's not clear whether the brothers have divided the Burgundian kingdom, but they each command part of the Burgundian army and they live with their war bands in different parts of the Burgundian kingdom. As the senior king, Gunther's seat is the Burgundian capital.
Their kinsman Hagen is not a king. He has no war band and is one of Gunther's vassals. His relationship to the brothers isn't clear, but it's implied that he is their half-brother, apparently son of their mother (because his father is Aldrian not Gebicca). However, in the Prose Edda, Hagen is a brother or half-brother on their father's side. In Thidreks saga, Hagen is the son of their mother by an elf. Etc. Anyway, for whatever reason Hagen doesn't have the status of king.
Their sister Kriemhild also has her own share of the Burgundian war band, which she takes with her to her husband's court. These warriors and her command over them are part of her inheritance. It's never made explicit, but the context assumes that their role is to increase her dignity and independence, and to protect her honor at a foreign court.
In this and similar stories, there are always rivalries and sometimes disputes among the different war bands when the kings get together. There is also tension between Kriemhild's war band and her husband's. In the end, Kriemhild's war band plays an independent role in the battle between her husband and brothers. In other words, they really are her own men, not just on detached service from her brothers' army.
The Nibelungenlied is set in the 5th century migration period, but wasn't composed in its current form until about 1200. It's certainly not history. Nevertheless, the social background is so different from 13th century Germany that most scholars think it preserves many elements of ancient Germanic and Norse practice.