Here is another thought about who else it might have been:
The lions, especially the way the Pride Lands are run, fit much more to human standards than to lion ones. It is always likely that there could have been inter-pride betrothals for political reasons much like European royalty is all interconnected due to power plays and all that.
So not only could there have been other cubs from Simba's own pride, but from other prides as well. So Nala must have really impressed Mufasa and Sarabi then...
...Or perhaps she isn't even from the Pride Lands and is indeed one of those union-through marriage things. Since we don't know who her father is...