Well, the only information we really have to go off of is the visual aesthetic from the teaser in the Roadmap from August, her being placed (alongside Morgana) in Demacia on the Universe Map, and Aatrox having quotes pertaining to her.
For Aatrox's quotes, I'll also have to bring up his quotes for Morgana, since the two Champions are pretty deeply connected, and Riot's almost certainly not gonna change that much.
First Encounter with Kayle
"Let me show you your error, Kayle. You are not worthy of ascension! You, are not worthy of the worms!"
First Encounter with Morgana
"You would bind yourself to this realm, Morgana? Fine! Let me show you the ground!"
Killing Kayle
"Die, Kayle, knowing your potential will never be reached."
Killing Morgana
"You disgust me Morgana. Die in the filth of your bound flesh."
There is an odd schism between the two quotes. Aatrox's words portray Kayle as striving to become something greater, and divine (through Ascension), with Morgana having bound herself to a lower realm, choosing to become something lesser.
That has some really odd implications for the Sisters' future lore, since it seems to depict them as not starting off as equals, despite their previous lore stating pretty much that they were just equal opposites. Angels, fallen angels, yadda yadda yadda.
Them being placed in Demacia, as far as anyone really knows, could just be a placeholder until they're reworked. Or, it could be something like Soraka's deal with her being placed on Targon, but only having the loosest connection to the mountain through being a Celestial.
So, the only really concrete information we have to work with is the teaser from the August Roadmap. A golden sword set against a firmament of stars, loosely coiled around by sleek, dark, magic chains.
The sword looks vaguely between Demacian and Targonian in design, with a closer thematic tie to Demacia (the fullering pattern on the blade suggests some semblance to wings, and we know that Demacia's crest is a winged sword), but from the guard down (or up, but, y'know, towards the hilt and pommel), the visual design kind of stops matching with anything we know.
As for the chains, the visual design looks strongly Targonian, with circular designs (albeit pointed in a given direction, which is slightly unusual) and each link is separated by a diamond-shape. Although, with Aatrox claiming Morgana to have bound herself to (this) realm, I'm doubtful that the designs are strictly Targonian, and may even be Celestial (the teaser does have a starry night backdrop).
There's a lot that I brought up that, looking at it, suggests that Kayle and Morgana might have a divide between them post-update, but, given Riot's aptitude for keeping the Champion's core identity in their updates, I can only say to take what I bring up with a mountain of salt. We can only really know when their update does come around, after all.