You might be thinking of this: The Stars, Like Dust, by Isaac Asimov. First published as a serial in Galaxy magazine in 1951, and later reprinted as a book. A good example of the "space opera" subgenre of science fiction. As I describe it, you may detect a passing resemblance to some of the plot elements of the original Star Wars movie. (But I don't know whether George Lucas had ever read this particular novel before he started making that film.)
The leading man is Biron Farrill, a college student on Earth when we first meet him. He is the son and heir of a man called "the Rancher of Widemos," the most powerful nobleman on the planet of Nephelos. Nephelos is part of an empire dominated by the Tyranni (so called because they are the human population of the planet Tyrann).
Over the last few generations, the Tyranni have gradually conquered about fifty other human-colonized worlds, and in many cases they allowed members of the local "royal family" (or whatever it was called) to continue "ruling," as long as they paid their taxes and generally submitted to Tyranni law. Rulers of different worlds have different traditional titles which the Tyranni saw no need to change. On Rhodia, it's "the Director." On Lingane, it's "the Autarch." (I wouldn't be surprised if that last one is what you were thinking might have been "Plutocrat" before it was translated into Hebrew.)
Biron visits Rhodia and begins to fall in love with Artemisia oth Hinriad, the beautiful daughter of the Director of Rhodia. (In other words, she is essentially a princess, although I don't recall if anyone calls her precisely that in the text.) Biron, Artemisia, and her "Uncle Gillbret" (actually her father's cousin, but she calls him "Uncle" since he's a generation older) all end up making a run for it in a small spaceship. Biron's father has recently been executed by the Tyranni because they had somehow learned that he was part of the inner circle of a group plotting a big rebellion. Artemisia is angry because her father has been ordered to have her marry a much older man who is a high-ranking official on Tyrann.
Gillbret thinks he can give them a good lead on how to find a world where the two youngsters will be beyond the reach of the Tyranni government. He has a strange story to tell about a time, decades ago, when he was the sole survivor in a badly damaged starship, and it somehow was knocked off course during a series of automated jumps through hyperspace, with the result that it ended up in orbit around an Earthlike planet which turned out to be the secret headquarters of a planned rebellion. Nothing overtly violent has happened yet -- apparently the rebel leaders are taking a very long time to make sure they have enough starships, heavy weaponry, trained military personnel, etc., to give them a favorable chance of winning when they finally make their big move against the Tyranni. Gillbret does not know the coordinates of this world he once visited (he was somehow returned safely home after his accidental visit), but a friend of his has a plan for ways to narrow the field in searching for that planet in a handful of star systems which might be where his damaged ship had ended up, way back when.
In other words, Gillbret is not an active member of the rebellion (and has a reputation of being an eccentric crackpot who shouldn't be taken seriously). But he could be called "a traitorous uncle" in the sense that he was going against the wishes of his ruler (Artemisia's father) by helping her escape from the arranged marriage to one of the Tyranni aristocracy.
Much of the book's plot involves the search for that world that serves as a secret rebel base (if it even exists -- the other characters only have Gillbret's unsupported word for it), with the Tyranni also hunting for it, and with various mysteries, accusations, betrayals, captures, and so forth happening on a regular basis to spice up the plot. In the end, Biron Farrill does some clever reasoning and figures out where the rebels have been headquartered all along. The rebellion does not begin at once, but we are led to believe that the time is fast approaching.