In my view they are not related. The directors's tone was joking when in some Disney event in 2004 they said Nala's father could be Scar or Mufasa whilst the animals in this movie are essentially human what with the concepts and practise of monarchy, laws, marriage and so on. I think it's much more likely they never bothered to decide who Nala's father would be as it's not important to the story but that it would likely be some rogue lion. Further proof of that is that one of the directors was recently AGAIN asked who Nala's father is, in Screen Junkies' Honest Trailer episode of The Lion King, and he did not name anyone. Not Scar, not Mufasa, not anyone. Not even though the Honest Trailer made a nod towards the potential incest. Therefore I feel it's safe to say there was no incest intended.

yeah there not