^ Good point, Sage.
That said, though, the lineage of most of the lions and lionesses in the movie is left pretty open. So there's several plausible theories about the origins of Sarafina and Nala. I'll try to delineate them.
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Sarafina was from another pride and came over with Nala. Here the premise seems to be that Nala and Sarafina are completely unrelated to the rest of the pride but Mufasa accepted them. Given Mufasa's nature, it seems probable enough, not to mention that it would add more genetic variety to the pride. If Sarafina was a princess, though, this would go along with the fairly common historical trend of betrothing the princes and princesses of one country to those of another to create peace. As someone pointed out, this would make Nala a princess and Sarafina a queen (or perhaps a princess herself), and they could have come as part of a treaty. True, this is never mentioned, but then, there are a lot of things that don't seem to be talked about in the movie, so...
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Sarafina is a part of the pride, but Nala's father is a rogue. In real life, this wouldn't be as plausible, considering that there's generally one or two male lions that mate with all the lionesses in a pride. However, the lions in this movie tend to display a more loving, mongamous structure of one mate for every lion. This is fine, except that it creates a notable sex/gender imbalance (seeing as Mufasa and Scar are presumably the only males in the pride, unless you count the books). So if she was born and raised in the Pridelands, then there was either another lion in the pride that was never mentioned, or she had a mate who simply wasn't a part of the pride. ...Or she was a mate to one of those two.
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Sarafina is a mate to Mufasa or Scar. Realistically, this one is plausible, but given the pains Disney took to avoid incest in TLKII, this one is probably not the one used in the movie. However, Zazu does mention that betrothal (ie, of Simba and Nala) is part of a tradition 'going back generations'. What does he mean by that, exactly? It could be a reference to Theory #1, in which those from other prides are betrothed to the sovereigns at Pride Rock - a trend continued with Kiara and Kovu's betrayal - but it could also mean that Sarafina was betrothed to someone in the royal family... presumably Scar, since Mufasa had Sarabi (who might also have been a princess from another pride, hence her 'different' looks). Yes, this does make Simba and Nala related, but it would also give a nod to the customs of traditional 'blue-blood' royals, who wanted to keep the title of sovereign within their own family. Given that people often make Scar Nala's father anyway, this one seems to be a popular fan theory, and it would make sense in a real lion pride... it is only within the continuity of TLK that it becomes strained.
In my own fanfic universe, I've played around with Nala and Sarafina's origins, since it is pretty open-ended. Generally I accredit light fur, brown tail tufts/manes, and green eyes to desert lions, and thus make Sarafina and those like her (including her siblings and other OCs of mine) from another pride. But this is by no means concrete. In my first fic, in fact, I made Mufasa Mheetu's father and added a whole relationship facet between Scar and Sarafina. But to be honest, I've kind of chosen #2 as my default theory, since it's the simplest and hence most likely to be true (Occam's Razor!). I would usually go with that one, though obviously any of these could be explain her origins.
Just my thoughts. :3