Empyreans supposedly have two deities for parents, though, or so some people say, despite being used as a default template of sorts. They're also a good example of high CR -- relatively bad in combat.
There have been demigods and similar reportedly powerful creatures with CR as low as 12-14. They're extremely inconsistent in lore, to the point of wizards of the coast accepting Aylin referring to herself as such. The reason given for those low CR is that they appear in campaigns that don't end in as high a level -- just like Bg3. While there are people with a very strong opinion on what the "right" CR should be, it's not established enough to be immune to handwaving. Additionally, again, Empyrean are actually pretty weak. Their best feature is the 22 AC. They're about on par in game with the Avatar of Myrkul.
Balthazar is not an impressive figure. He should be incapable of most of the feats he has accomplished, to the point that even the capacity to capture something as a lowly deva seems personally unlikely. Again, handwaving. Several characters are not the level they appear as in game. If plot armour is needed, they receive plot armour. If plot *weakness* is demanded, they're about to roll a long string of 1s. You can about bet someone in the development process wanted a "daughter of Selune", realised: "oh shit, that's way too powerful" and out came Aylin, who's just a... something.
Once you start applying deeper logic to the enemies and creatures in Act 2, about the entire plot becomes nonsensical. Even Ketheric is just a one-smite pony. Aylin may or may not be an Aasimar who is singularly blessed by Selune, as the "Child of the Moonmaiden" passive suggests. She could be a direct child of Selune, somehow, or a proxy child, or one of several weird options. She could even be a derranged Deva, or something completely new and singular. Her power level in game could be accurate or extremely downscaled for the campaign. It's Larian being Larian, as usual.
