@Riot: How to the Angel sisters fit into the current lore?

The link above leads to a year-old thread speculating about Kayle and Morgana being Darkin. While I wouldn't go as far as fully accepting the option, I still wonder where in Runeterra Riot want to fit them in...

Since the "summoned from a distant world" argument is not valid, Kayle and Morgana are almost bound to come from Runeterra. There are, however, a few thigs that make them inconveniently unique:

1. Their visage is unlike any other's in Runeterra. The two are indeed polar opposites, but at the same time they look uncannily similar (or at least would look like to a common Runeterran, if they ever appeared to one). The closest being to appear visally similar is Aatrox, and potentially other Darkin.
2. They are divine beings. They are more powerful than a common denizen of Runeterra. However, the source of their power is not well explained in the lore. Could it perhaps be tied to the same font of power from which Demacia and Noxus draw?
3. They have strongly developed rivalry for each other. At their power level, there are no such beings as powerful as them that would have such developed diametrical opposition. The closest comparison would probably be Brand and Anivia, if they even keep their old lore.
4. The last reason is much more subjective, but to me, Kayle and Morgana don't really look like they come from Runeterra. They seem alien, considering the art style League of Legends is currently heading for. While most other old champions recieved plausible explanations for their looks, or had neutral enough visage to blend into the new world easily, the Angelic sisters still lack any acceptable reason for their appearance. One other champion still suffers from this problem imo, and that is Shaco. However, his case can be solved much more easily with the presence of the Shadow Isles and especially the existence of Karthus (note his and Shaco's color schemes).

Considering all the precedent arguments, I feel like the sisters need a good place where to fit. I thought up a few plausible locations:

1. Demacia and Noxus – The dichotomy of the two city states could easily serve as a backdrop to explain the sisters' history.
2. Mount Targon – If approximated to the powers of the Sun and the Moon, Kayle and Morgana could serve as avatars of the celestial deities.
3. Darkin – In similar vein to Aatrox, the sisters might be another two Darkin to the collection of five. The warlust of the races could explain the bitter rivalry between the sisters.

I ask then, Riot, who are the Angelic sisters supposed to be? I will be thankful for any insight you might provide.