That's where Mach's principle comes into play. If there were a universe in which only the two rockets with the twins existed, and nothing else, then I don't think their clocks would change pace with respect to each other. But the total inertia present in our current universe makes a frame of reference in itself. At least that's the way I understand it, but I'm no relativity expert, so maybe I said something stupid.... Mach's principle is something that fascinates me.But if it's all relative it shouldn't make any difference whether there's an Earth or Sun around.
That's why I think it also involves acceleration of the two bodies.