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.
I don’t believe the shooting script gave their ages. The previous draft of the script gave her age as 16, but not the shooting draft. Promotional materials list Luke as 20 and Leia as young.