i think it's so interesting that you conceptualize babs as their brother when like the most important part of him (to me!) is that he's NOT their brother and that therefore he's the site of ... like everything that they can't manage around each other gets displaced onto him specifically in the context of: he's NOT an heir. he's NOT a necromancer. Thee essential babs moment is corona's "and who wants babs when she could have had me!" - obviously yanth would never ever have used corona as a battery, because she loves her. but babs? she doesn't need to care about him! if he'd been their brother, she might have had to find some way to care. he feels to me like the very gothic like, governess or poor relation, someone connected to but nonetheless easily disposable by the spooky house.
anyway sorry im just obsessed with them it's so good. bite chomp chew.
yesss i think this is such a good point too, he exists in this weird precarious position where he was raised alongside them (hence the brother thing) (and also i think the ease in which they know each other and interact with each other is very sibling-like, like ianthe rolling her eyes at the idea of someone marrying him for example) but he fulfills a specific purpose and wouldn't be with them if it weren't for his position as cavalier. i like your comparison to gothic fiction bc tlt has such big gothic romance vibes so i think that is more apt....though i do think that him being a cradle cav indicates he probably comes from a higher status, similar to someone like marta who came from a respected cav family
also ignoring what i just said, i think the fact that he's a man plays a part in his separation and the way brother/sister relationships work differently from sister/sister—a cav, pre-lyctor, functions to protect the weaker necromancer, which feels very gendered to me in the third house's case, and sort of plays on the protective big brother/younger sister fictional archetype in a way that obviously gets crazy subverted. and also there are dynamics that exist between ianthe and coronabeth that are so gendered that naberius probably can't relate to as a man and non-royalty that would separate him from them no matter their relation. third house dynamics are literally so tasty i cannot wait to digest this shit even more