The dialogue referring to it, is there more?
(CAMP_Nightsong.html)
Player: Are you really Selûne's daughter?
Nightsong: Do I not radiate with my mother's brightness, her glory? There can be no doubt: I am of her silvered flesh, her celestial womb.
Player: Would your mother be willing to aid us in the fight against the Dead Three?
Nightsong: Why, she already has. She has brought her sword to your side. Dame Aylin. (devnote NodeContext: referring to herself. She's dead serious here.)
So mighty are her wonders, her great wisdom. Together, we will set this fair land free of tyranny and murder.
She may believe something false or lie but the examination tooltip should say the truth, it's annoying…
Yeah that's what I remember, it's like she's trying to convince herself more than me by stressing how much she embodies Selune, which again, would be equally true for any regular Selunite Deva. Like, girl... did Selune actually raise you as her one and only daughter or not?
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.
I suppose it also depends on which divine rank we're talking about, considering Selune is one of the Greater Gods. A CR 12 direct child of the Greater Gods is certainly not something I personally remember from any 5E campaign, so that should prove an interesting topic. You also raise a salient point regarding Balthazar - How in God's name ( pun intended ) does some schmuck necromancer from the Sword Coast capture THE one true daughter of the Greater Goddess Selune in the first place and then drag her back to the Shadowfell for the next 100 years, exactly? That's like one degree of separation below Karsus' Folly itself, way more cosmically impactful than simply capturing an elder brain. Sorry, I don't buy - as you say, even his ability to constrain a deva is in question, so I'm going with the 'Aylin is a deranged deva' theory.
We're also still left to contend with the fact that Aylin *looks* precisely like a Deva, including the in-game model of a Deva, and has almost the exact same abilities of a Deva, including the Devas summoned by Planar Ally in-game. This begs a philosophical question: Even if she was, somehow, the one and true only first-generation child of the Greater Goddess Selune, and given that she does not seem to possess any power greater than that of an average Seluinte deva, let alone Planteras and Solars, nor any indication of a stronger relationship/communication channel with Selune ( Who left her to rot in the Shadowfell too btw, RIP ) than any of the former, then what difference does it even make to dissuade us from calling a spade a spade?
Deities aren't corporeal beings, which means that both divine offspring and divine servants are manifested into being. If Aylin is a CR12 child of Selune that has all the powers and appearance of a Deva without any additional observable perks, then for all intents and purposes she is a Deva.
P.S - You're right that Empyreans are pretty weak against medium-levelled parties, but they do have much higher HP than Aylin, much higher ability scores and legendary actions, so ehhh. Kind of a fuck-up from WOTC as well right there, they're supposed to be super-powerful when DM's consult CR to design encounters, even more powerful than a Solar, but are functionally not. CR 30 Tarrasques also auto-lose against anything that can fly, run, and employ ranged attacks.
