Heard from some people on the forums that there is some sort of an incistuous relationship between some people in the Battle-Born clan. Does anyone have any details/evidence for this or is it mere speculation?

sounds like SC propaganda, I have seen nothing in game to support such a claim

Besides the fact that two of the family say they're brother and sister, but the game registers them both as parents for small-child-Battleborn.

(I'm terrible with names)

Was a thread on this some months back, was probably a mix up in the CS and nothing more. I think her mother is referred to being elsewhere as well.

Pretty much every single Nord in Oblivion was inbred, so it wouldn't surprise me.

That information is only on the wiki, not the uesp. The chart on the uesp makes it seem as that one guy married into the family.

There's no credible evidence to support this. If anything, it's a mistake in the game data with the relationships, like others have alluded to.