Actually, Luke and Leia were respectively 20 and 16 in the shooting script for the 1977 film. So they weren’t the same age, and thus not twins.
However, nobody said anything about them not being related – in fact, the instances of brother/sister incest in both Wagner’s Die Walkure and the Arthurian mythos involve half-siblings born at different times. But that might be a little more than you wanted to know.