Actually, Kay, Siegfried's parents are full brother and sister. (It's Brünnhilde who is their half-sister.) One might be tempted to blame it on ancient mythology, but actually in the Siegfried myths with which I'm familiar, Sieglinde (or however the character is named in a particular version) is rescued from her marriage by her brother, but has a baby with someone else (a king she eventually marries, I think). Wagner, as ever, tightened the situation and eliminated/combined characters.