Nobody knows who Nala's father is, and I doubt it's Mufasa, or Scar.

Sarafina probably came from a differnt pride, and was already pregnant with nala when she came to mufasa's pride. she cold have mated with scar, i think that's how she had mheetu. but, i don't think that nala and simba are related. to be honest, i really, highly doubt that. plus, it's just a movie. they probably wouldn't make it "realistic" so srafina could ONLY mate with mufasa or scar.

That indeed is option. But nothing is known about Sarafina and Nala before movie story. Including Nalas father. So there are more options, there could be more males in pride before movie and one of them could be Nalas father. Could also be that Sarafina joined pride when she was already pregnant with Nala. Some do believe that Nalas father is rogue lion. Possibilities are endless, and there is little to no prof about which is real and which is not. Some are based on natural loon behavior, in which case both Mufasa or Scar could be Nalas father, since lions care little about mating relative. Some base on fact that this is Disney and relatives should not mate, like story of rogue lion, in which I believe. And some also try to find her father by appearance.

It's fifty- fifty really. I'd actually say that they aren't because in certain places it's against the law to
marry someone too close in relation to you. Only the rulers can marry their sisters and brothers, so they could be. You never know, they just might.

It is highly likely that they are half siblings however there is also the chance that they are not. Although a pride typically consists of one male lion, there is a rare chance that a coalition, a pride that consists of multiple male lions of up to four. It could be that there might have been a former coalition between Mufasa, Scar, and some other male lion. Gestation for a female lioness is around 4 to 5 months so anywhere between that might have been when the other lion could have been ousted and left Scar and Mufasa to run the pride.

Despite that fact, the similarities in appearance between Simba and Nala suggest that they are half siblings.