No wonder they have so many kids.

She's just like her mother... I mean seriously, how is Commander Bumi exactly like the original Bumi while Aang is his father?

Well, some people in the water tribes have darker skin (don't ask me why, since they are originally from arctic regions...). For example, Kya is Tenzin's and Bumi's sister and she also has darker skin. So someone in Katara's or Aang's family, somewhere along the way, probably had dark skin and it is a latent gene.
And we know that Aang DID have hair, I even think it was more or less Bumi's hair color.

Sokka being the father would make Lin and Tenzin's past relationship kind of weird, since they would be cousins....

That's one of the many rumors...

His skin color and the general fact that he has hair has nothing to do with what I was talking about but good to know...

Oh, I thought you meant his aspect. Did you mean his features, facial lines... and so on? He isn't thaaaaaaaat different from older Aang in that aspect, and he could be said to resemble Katara's family, like his father.