Actually Tegan's book doesn't really mention much of his past except for his first breedmate Sorcha (I think that's how it's spelled). Considering though that the mountain range in the tapestry from Lucan's room (That Kassia made for him) is in Prague and that it was also said in the book that during the time of the war with the ancients they were in living in the area that is now Prague, I am going to make an estimated guess that Tegan is also from Prague (Lucan and Tegan have been friends since the beginning).
As for his mother, since his father was an ancient, I am going to assume that Tegan's mother is no longer alive (either by the ancient's hand or because once he died she could no longer live an 'immortal' life). We also don't know exactly when all of this happened, so Tegan's mother could be from celtic origins...maybe captured and brought to Prague or something entirely different. Just a bunch of guesswork from me as I just finished reading Tegan's book again.
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Ignorance killed the cat, curiosity just got framed.