Kiara is about a month or two older than Kion in the alternate story. As she calls him "Little brother". In the two original films, Kiara is the only child as she and Fluffy are the same.

I never cared for TLG. to me it's pointless. I only stick with TLK and Simba's Pride, even TLK 1 1/2. TLG has awful animation and the voices are different. I seen the return of the roar and I didn't like it. The creators of TLG should of thought on something else for a kids cartoon and leave TLK franchise it the way it was before.

Disney still say that Kion and Kiara are the same age.
TLK 2 is not canon. In TLK 1 Simba and Nala have only one child Prince Fluffy. Scar no children, lions supporters.
TLK 2 and TLG are so canon as SNA

It's very confusing, but given Kion's visual character design, he looks about as old as Kovu looked in the sequel, so I'd estimate that they're twins, with Kiara being born first. Kiara doesn't look old enough in TLG to be five months older than Kion.

It's impossible for them to be a month apart in age because lions are pregnant for 110 days.

Remember that Simba and Nala's cub in the original movie never had a canonical gender. They were just "Fluffy", not "Prince Fluffy". Some Disney-labelled (though not Disney created) books decided that the cub was male, and others said the cub was female. It didn't matter, basically.

It's probably worth noting that, technically, while TLK2 and TLG are not considered canon (the original movie wasn't made with either of them planned, and they didn't come from the creators of the original movie either), they are closer to canon than SNA, which didn't even come from the Disney company (they just distributed it). Those stories also had a *very* limited audience (US only) and are only as official than any other Lion King book with Disney's name on it (I'm not sure why people kick up such a fuss about SNA specifically, since there are loads of other Lion King story books that seem much lesser known even though their just as official/unofficial). At least TLK2 and TLG came from Disney specifically and didn't just have the name stuck on them for distribution purposes. But neither were animated at the main Disney studios.

I like the idea Kion is the older because it is very similar to Fluffy

I second glelsey. It's impossible for Kion to be anything but Kiara's twin/litter mate.

Also, why is older even an option here? The pilot episode clearly has Kiara call Kion her little brother.

And, Fluffy can be seen as Kion even though he was born after Kiara. Because of course they would have presented all the royal cubs and not just the first-born.