What's your theory for the SP/TLG timeline?

[Rollo]

^ But how do we know Kiara's not a teenager in TLG? If you're going by the size, then you should probably know that's a bit redundant because TLG has messed up the sizes of the characters considerably. Take Simba for example. And Pumbaa is the size of a cub Kion's age now. Both of these things are different in the original movie.

I don't consider her a teenager in the latter half of SP. I think she's a young adult there, which gives more of an understanding as to why she'd be so annoyed with her father for being overprotective, if she was, say, in her early 20s in human years. Kiara has always been playful and naive. People can act a lot younger than they actually are...I always thought that was the case for Kiara, and that's why it's such a surprise when she ends up being the one to deeply understand the moral of the film.

[SimbasGuard]

If Kiara was a teen in The Lion Guard she would at least be the size that Nuka was in the first part of Lion King 2 or at least what that size equivalent would be for a lioness. In Return of the Roar, Simba stated that Kion was still a cub. Kion and Kiara are the same size, now size inconstancy aside. I doubt the animators would make a lioness that is intended to be a teen the same size as a cub.

[Troll Berserker]

I did not say she was a teen in TLG, just older kid than in SP (maybe a preteen, I dunno, like 12-15 in human age while she looks 6-8 in SP).
If she was to be older than Kion (not littermate), she would be a teen or young adult and noticeably bigger/older than Kion. But she isn't. That's why I think Kiara and Kion are littermates. If Kion was to be born after Kiara's cubhood events of SP, he would still be a baby, but in the letest episode he's shown to be older than "young kids" (Mtoto and his friends).
Let's compare our lion siblings to Mtoto&friends. They are little kids who only spend their whole days on playing. Same with Kiara in SP. Simba talks to her about her being the future queen, but she doesn't care much. In TLG she starts taking her future role more seriously and instead of playing she spends her days in learning. Kion wasn't meant to have such important role, so he was shown being more immature, but not necesarily younger. Simba trusted both Kion and Kiara with reposnsibilities they can now understand and handle. So to me both Kion and Kiara appear to be the same, preteen age, when you can still say they are "still kids", but they start to have responsibilities.

[Elton John]

Don't lionesses age slower than males? That would make sense as to how she is older than kion but still the same size.

[Squeely]

Hair tufts could have some genetics to them, too. What if Kion's hair is coming in a lot more quickly than cub Simba's did? As for Kiara's size, that's difficult to use as a way of determining things, because the sizes of already-established characters are all over the place, and the show is heavily stylized. I think the possibility that Kion was born sometime after Kiara could be there. It's difficult to say, and we won't know for sure unless we're given an official explanation.

I do think TLG takes place after the first half of SP. Probably a considerable while after, as Kiara seems to have had quite a bit of time to grow. I don't think her character is different so much as she just changed between SP and TLG. Started taking becoming queen a lot more seriously.

[Ashera]

'You’ve seen Lion King 2. Kiara meets Kovu and they get into trouble. The very next scene, Kiara is fully grown and off on her first hunt. Clearly there’s a few years gap in there, so we’re filling that gap with The Lion Guard.' - excerpt from the interview with Ford Riley.

Source: http://tvdaily.com/exclusive-interview-disneys-the-lion-guard-creator-ford-riley-talks-new-lion-king-sequel/

[SimbasGuard]

Well now thanks to Ashera, we now know That The Lion Guard is set within the timeskip of Lion King 2. Now we know that Lion King 2 will (Most likely) not be retconned away by The Lion Guard. Now the only retcon that needs explaining is Kion's existence. Because I also think that it is more likely that he and Kiara are littermates.
Also it is neat to know Fuli is "Unofficially" 2nt in command of The Lion Guard

[moonsugar33]

Ach, that blows my headcanon that Kion and crew are out of the Pridelands/possibly deceased during the entirety of TLK II right out of the water.

[Aelwen]

From the realistic point of view Kiara and Kion should be littermates. We might even imagine that Fluffy presentation was actually Kion's (Rafiki put Kiara in the cave and took Kion), it also couldexplain their size differences: after six months male cubs overgrow their female littermates. If Kion is really from the second Nala's litter that he cannot be less than 5 months younger than Kiara and such difference would be visible.

[Squeely]

So I was right! This does take place between the first and second half of SP! Awesome.

I still wonder if we'll ever get any word on whether or not Kion and Kiara are littermates. I could still see it going either way. Having Kion born later clears up the issue of where he was in the first film, but having them be littermates explains why they're roughly the same size (though I could see it being a stylistic choice to draw young lions roughly the same size). Guess you can't have a TLK sequel without plotholes!