Prince Oberyn "The Red Viper" Nymeros Martell is a member of House Martell. The Martells are one of the Great Houses of Westeros and the Ruling House of Dorne. Dorne is one of the seven regions of the Kingdom of Westeros, which means that they are under the Rule of the Iron Throne.
My question is why Oberyn (and also his siblings) called Prince (or Princess) if they are supplicants of the Iron Throne?
We don't see the Starks called Prince [A Clash of Kings Spoiler]
Until Robb Stark styles himself King in the North and Bran/Rickon are called Princes
So, why are the Martells called Prince?