One question I should ask myself, as I guess nobody will actually ask me anytime soon, is why I did not conceive Urando as a counter-Earth instead of a terrestrial exoplanet? At first this would save me much trouble of imaging a planetary system which is for all practical purposes similar to our own Solar System and more importantly it would absolves me from the need to devise some type of interstellar travel, something I usually avoid in my mundane science fiction stories.
There are two reason not to move Urando to a counter-Earth. Though a counter-Earth planet would not be visible directly from our planet, it by now way completely undetectable. A counter-Earth would be detectable through gravitational influence on other planets, and not only on Earth.
The second, and in my opinion more important reason, is related to the level of technological development of Urando, or at least of its major nations. As a general rule both Gramatia and Benya are fifty years ahead from our civilization. If Urando was a counter-Earth, these two nations would definitely have launched a war against our planet. Consequently I would have to explain why these two countries did not start a massive war.
So having Earth and Urando a hundred light-years away from each other and only connected by a single wormhole, provides me for liberty to set up a world. The wormhole made it possible for Urandans to go to Earth in virtually no time (a couple of days is nothing in spaceflight), but the entrance is hidden and has only recently been discovered by Urandans. It puts a severe restriction on the ability of Urando to carry out a massive attack on Earth.