Okay so I have a very guesswork theory on the father of Lin Bei-Fong
So the father of Lin Bei-Fong actually walked out on the family when Lin was very young, thus why in the 1st season she doesn’t mention him AT ALL.
And because we don’t see Suki whatsoever in the first season or have her mentioned, I am ASSUMING she didn’t get with Sokka for whatever the reason.
THUS (being the nice guy that he is) Sokka became the adoptive father (per-say) of Lin Bei-Fong.
(Also I know that the whole father walking out on them is kind of rough and dark, but the show is seriously somewhat dark, so it would go along with the theme.)
