I agree with sno on this one. A second, more far-fetched theory is that any siblings Simba or Kiara had might have been born after their presentations but died before they could reach cubhood. It's actually very common for newborn cubs in Africa to not live past 2 or 3 months because they're simply weaker than their siblings (in this case, either Simba or Kiara). Part of the Circle of Life is survival of the fittest, after all. But if that were the case, you'd think they might have at least been mentioned in the two movies.

So as I said before, this is quite far-fetched.