I did read in the interview regarding rebellion and they did touch on the prospect of the developing Sins 2. While the guy who was being interviewed didn't openly admit that Sins 2 was in development or not. There was a response that hinted to a Sins 2, but not actually admitted to, when questioned about the feared 4th race that has the Vasari on the run. Actually wait one minute... It will be easier to quote it.
"GameSpy: Why did you decide to split the factions into two pieces each instead of just creating a fourth race?"
"Brian Clair: That's a good question. For a fourth race, we didn't really want to go there until we get around to a Sins 2, whenever that is. There are certain things, storyline elements, that we want to introduce in a full-blown sequel, instead of something like Rebellion, which is kind of an in-between point. [Note: Stardock stresses that this does not amount to an announcement of Sins of a Solar Empire 2. Sure sounds like they're thinking about one, though!] For this title, we want to continue along the established lore timeline."
As you can see. Rebellion is not Sins 2. But there is a possibility that at some point there may just be. Stardock, speaking at least for myself. I really hope there will be a Sins of a Solar Empire 2. And I would love to meet this feared 4th race that has the Vasari on the run on the battlefield. In other words, if you "were" to make a Sins 2, you can count on my custom and I'm sure there are others here that feel same as I do.