What is really RT Dynasty? Just one starship and characters on it? Do PCs answer to some Dynasty Patriarch? Do they have siblings flying around in starships of their own? Are there some imperative on PC Rogue Trader to marry and provide the heir?

I don't know why but today for the whole day I couldn't stop thinking about RT Dynasties and comparing them to Dune Great Houses.

What do you think?

Dune's Great Houses are a good analogy, and extending that analogy, the Warrant of Trade is sort of like a CHOAM Contract, albeit with greater, far-reaching powers. Rogue Trader Dynasties can have their Warrant of Trade as a foundation for their bloodline.

In general, it's assumed that there is one active Rogue Trader who has the Warrant of Trade and who is played by the PCs. His children or siblings may be aboard the ship or somewhere else entirely. Many RTs send their offspring to pursue Naval careers until they take over.

As for the number of ships, it's not spelled out directly in the rules. There's one main ship that directly belongs to the characters and is created by the Starship rules. Other than that, I personally assume that most Rogue Traders are either affiliated with one or more smaller trade cartels or own a few freighters to do the milk runs - after all, those exclusive trading rights you have with the people of Ake-Mosc won't do you much good when your only ship is busy engaging in other endeavors like the next gaming sessions.