Prince/Princess

Directly under the Queen or King come the Princes and Princesses of the Kingdom. Only the direct offspring of the ruler are Princes or Princesses.

When one of the Prince or Princess siblings rises to the throne, the remaining siblings keep their rank, although it ends with them and is not passed down to any children they have (unless they should end up taking the throne and becoming King or Queen).

Thus you can have up to three or so generations of Princes and Princesses- the Aunts and Uncles of the current ruler, the siblings of the current ruler, and the children of the current ruler.