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Simba, Nala, Kion, Kiara © Disney
Kiara wasn't the firstborn.
That cub, from the end of the 1st Lion King film, is a boy and Kiara's presentation is only at the beginning of the sequel.
Here are links that explains the difference of the cubs from 'The Lion King'.
Besides, someone on YouTube made a video that also explains the difference of the cubs.
Anyway, I like the picture.
Only reason the presentation and design are different now is because different animators and the fact they didn't want a Simba clone most likely, hence why Kiara isn't a exact clone.
Just my two cents
But Disney still hasn't mentioned anything about him. I think they just came up with the idea of giving Simba and Nala a cub but didn't created a design for him/her at that time. Later they created Kiara. But originally it was going to be her, I guess.
Officially Kiara is the firstborn (at lest if we talk about her and Kion) and that is said in the TLG series.
In my theory the cub presented in the first movie is not Kiara as well and I've mentioned that. It's Kion. But still it's just my idea why the cubs look different not a canon version.
And I like your theory, it makes sense =3