Here's what we know:

-The shots of Fiamet's room in Act 3 we've been shown have her with an Earth globe and her cocoon is on top of a human bed. She has definitely been exposed to human culture and lifestyle.

-Grandpa Harley's mansion, conversely, contains an Alternia globe. It's suggested pretty heavily that Grandpa Jake has used the portal to go to Alternia and do big game hunter adventurer shit as you'd expect from him

-Joey, obvs, is one of Jake's many children he abandoned and she is the main character and it's her story

-In Hussie's Hiveswap planning notes (which also contain Fiamet, she's a foundational character for this story) it mentions that Joey wishes she had a sister

Idk I think the writing is on the wall here! I'd be pretty shocked if this wasn't the main thrust of the plot.

(The recent reveal that limebloods were pyrokinetics, which figures into the lime Fiamet's green-fire-breathing axolotl lusus, could maybe connect in some convoluted way to Jake's eventual acquiring of Bec who has green fire First Guardian powers... but that's a bigger question)