Did you ever get an answer to your question?. I too am playing as dorne and haven't found a way to recruit the sand snakes

The one you See at court (obara Sand) works like a Lady. The "Real" Sand snakes can be found at the watergardens. I Heard you need to choose dornish as starting cultute in order to hire them (same as direwolf=north or giant=free folk). Under certain circumstances you can change your kingdoms culture, i don't know if this has any effect on recruiting faction specific troops. I am quite sure you can only have one child via marriage, never heard about a second child or of Bastards

Somethings require you to be at a certain level of renown and have a certain amount of relations with a faction in order to trigger. So, while being Dornish, you may still have to increase your relations with Dorne or increase your renown.