They're Álfr. They work in other ways as humans and also MC didn't share family details either. Medb and the Callister brat were bonding, sure, but that doesn't mean they have to talk about their love lives and their family. After all both are trying to find out if they can trust the other person.
Again it's all about Medb. Why am I the only one concerned by Finnabair in her own right?
Also, the MC didn't share his family details because everyone was already well aware of them - both who his parents were and the fact he resents them concealing the Templars from him.
Finnabair a daughter of the sun like all Ljosálfar. Medb explained that. But for the person who gave birth to her and is the biological mother it's Medb. It's literally said during the ceremony.
We're told offhand that "Ainu" means daughter, but translation is never as easy as a one-to-one comparison. Words, especially basic ones like 'daughter' can have a lot of connotations that don't map directly from one culture to another. And even in English, daughter is often used to mean things other than a female, biological child. Just because Medb uses the word Ainu (especially in a setting as prone to euphemisms as
a funeral), it doesn't automatically mean she gave birth to Finnabair. But it does seem like Mab's
eulogy describes Finnabair specifically as the daughter of Ailill and the daughter of Medb, so in this case I think the literal implication is warranted.
I still don't see why that revelation should limit access to the thematic end of the whole chapter, though. Medb isn't the primary character in the game, yet there's very little room for error to see the scene without following her path, and zero margin for error if you want to see the scene
without getting to the point of a potential Medb will consider a future romantic relationship. That's just bonkers. And if there IS some strange issue with learning Finnabair's genealogy, at least make an alternative version of the scene where the MC can watch from a distance, too far way to hear the dialog. We need *some* sort of closure for
the character who actually died.