Good question. As far as I'm aware, the only universal taboo is Mother/Son (but I don't know whether that claim's historical or theoretical). From what I know from research into the Fred West murders in the UK, Father/Daughter incest, though illegal, was in practice in parts of rural England into the 20th Century (on the basis the father would ensure a "good first time". Some of the incest claims were later disputed by West's brother). As far as BtVS is concerned, I think Spike/Dru and Angel/Darla allude to Mother/Son incest (Dru even identifies herself to Anne as "the other that gave birth to your son" in LMPTM). Beyond that, I think it uses the common-or-garden Oedipus complex (with a side-dish of Totem and Taboo) to excellent effect (but I have a tendency to read that metaphorically). Angelus (who sleeps with his "mother" and kills his actual father) is the one who comes closest to fulfilling childhood fantasies.Although, for a show and a fandom that puts so much emphasis on "chosen family," what is incest, anyways?
I don't think Donnor disturbs family structures inasmuch as family structures are psycho-social but nor does it add to them. It's just a bit...meh.