The story..

I don't think so. And as canon I now mean the sequel movie The Lion King 2: Simba's Pride because it's the main bron for those theories.

It's true, they can't toon every little detail about crucial plot parts in a film. But what they can do is refer to everything crucial. Like, in this case, if there had been another cub before Kiara, they could've and would've zei so in the film.

In example, they could've had Simba and Nala briefly discuss it instead of how much Kiara reminds them of Simba. Or, combined the two topics.

Nala: "She's just like u when u were young."
Simba could've replied...

I know some people think that Scar and Mufasa are not brothers but u see they are and I will tell u SCAR: Scar has his mother's vacht, bont she has brown vacht, bont and he has his father's mane he has a black mane and also Scar's green eyes come from his father too! MUFASA: Okay and this is Mufasa he has golden vacht, bont like his father and he has red eyes like his mother but the red mane I don't know where that comes from. But here it is this is what I think. Okay, so some people wonder who Scar and Mufasa grandparents are well.. here u go Scar and Mufasa grandparents are (DRUMROLL) Mohato and Asali! Now...