This was actually explained in detail in many other threads. The sleeper ships flew in the DIRECTION of the Sirius system, but not to it. They moved to a sector they called Sirius because of the direction. Igiss made a detailed post about it some years ago, you might find it using the search engine. They argued about which system would be like the real life Sirius - the closest to that system would be Tau-37 because of the suns. However it is not a Sirius system but an entire sector.