Question: I am reading Dune at the moment and can't work out why the Atreides line at the beginning of the book seemingly contains only three people, the Duke, his Bene Gesserit consort and son (later it turns out to be four including a daughter). No mention is made of other relatives, which would be expected in most House families (especially one as old as te Atreides is supposed to be). It may be answered that the Bene Gesserit kept down the numbers in order to control the their genetic traits, but is the line really that open to obliteration? It seems strange that in the whole of Caladan there are no relatives to fight for / take up control of Arrakis after everyone thinks that these three are killed. Even Baron Harkonnen had several young relatives to choose from.

Another related question, why do we hear nothing of Caladan after the Atreides leave? Why don't any of their subjects try to avenge the deaths of their Ducal line?

Any help, either from within the Dune Universe or from a narratological viewpoint, greatly appreciated!

Answer: 
The impression I got was that aristocratic breeding was tightly controlled by the Bene Gesserit. And as for Caladan- what could they possibly do? If they tried to rebel against the new Dukes, they'd be stomped on by the Sardaukar. If they tried to go intergalactic- the Guild would refuse to transport them.
They know that the whole thing is a set-up and the Harkonnens must have planned their deaths in detail. But being small and unimportant on caldan and having a big and violent personal enemy they have no real future. It is inevitable that caldan will fall to the Harkonnens some day. Then they are offered the most important place in the universe and they know that their only chance for them (and perhaps for caldan, too).

I think everybody knows that their chance of surviving there, is nearly zero. But they know that. Still, their possibliity is to enter the obvious trap that has been set-up for them proudly and knowing that it is a trap hoping for a complete miracle.