We see Elia Martell being given the title Princess, but this was due to her birthright as the daughter of the Princess of Dorne. Rhaella was a princess by birth, her parents were king and queen, and so was Shaera. But why wasn't Betha Blackwood given the title Princess before she became queen consort, when she married Prince Aegon "Egg"? Dyanna Dayne, wife of Maekar I, died long before her husband was crowned, and her title was simply Lady. Will the wife of a prince be called Lady until her husband becomes king and she is crowned queen consort?
You are correct in your assumptions. In Westeros the only ones to take on the style of Prince or Princess are
- Children of the King
- The heir to the Iron Throne (Prince of Dragonstone, not styled after the Targaryens were disposed)
- Rulers of Dorne (allowed to keep these styles after Dorne peacefully joined the Seven Kingdoms after resisting the Targaryen invasion)
- Children of rulers of Dorne
We see no other Lady use this title elsewhere in the nobility of Westeros.