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: 
It is established that the Atreides have been a very poor House for many generations, with its renaissance only really beginning under Duke Leto's grandfather. Leto's father was married to a Corrino princess and the family enjoyed much success under his rule. It is not uncommon, historically, for royal and ducal families to die out due to a lack of heirs, and the Atreides are treated as being "not very fecund," meaning that they have never had many children. The Corrinos, genetically very closely related to the Atreides, are in the same boat; Muad'Dib's claim on the throne was actually almost as great as Shaddam's even before he married Irulan. 

The Harkonnen's on the other hand, are portrayed as being a very large, fecund family; the Richese ducal family, the Harkonnen's closest allies, are portrayed as having more than ten children in just one generation. This is why the Harkonnens have survivors to go underground after the fall of the family, as seen in *Heretics of Dune*, when the Bene Gesserit discover a post-Jihad stronghold of the surviving Harkonnens on Giedi Prime. Also bear in mind that it is very common, as is seen with Irulan, for the daughters of noble Houses to marry into other noble Houses; after this, their children will be members of the other House, rather than the one they were born into; this leaves the Atreides with many relatives, but no other members of the House itself.

Regarding the Caladanians; Duke Leto took most of his military with him to Arrakis to defend against the expected Harkonnen-Sardaukar attack. Therefore, there was very little military presence on Caladan to fight back when the Imperials and Harkonnens showed up. Also bear in mind that from the point of view of the Caladanians, the Harkonnen takeover was legal; they were not aware of Sardaukar involvement. Their former House's hatred of the Harkonnens is unlikely to have rubbed off on the general populace, and they would not have had a very great capacity for resistance, being a simple people.
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.