I think I might finally need to stop tacking things onto Rocl's posts via PM and actually dive in on this one, lol.
The game gives inconsistent hints about who Yotsuyu's mother is.
Asahi's mother directly states that Yotsuyu is her sister's child. However, in the Japanese text, Asahi's mother (named in the script code as YOTSUYUSMOM) uses the same bumbling pauses and awkward laughter while saying that Yotsuyu is her sister's child as she does when lying about her being too fragile to attend school (yet somehow perfectly capable of spending all day in the fields). However, in both German and French Yotsuyu goes out of her way to refer to Asahi's mother as her adoptive mother. Yotsuyu may call her "mother" (and Asahi "brother"), but this makes sense either way.
We can set this aside for the time being, as it's rather moot by the end of things.
Many Othardian / Hingan conventions are Edo-ish period, but everyone and their mother having a surname wasn't common in Japan until more recently. It's likely that they're just all of the Naeuri clan, while Yotsuyu was married into the Sashihai clan to increase the Naeuri's status. (Notice what Gosetsu calls her while trying to rile her up: Orphan of the Naeuri, Widow of the Sashihai. It just fails to anger her because she's not denying or hiding her shame, she harnesses it for strength; she sees Hien as pompous and ignorant and out of touch with the commoners while she knows the true Doma, and the true Doma was hell on Hydaelyn.)
The game is also inconsistent about how Yotsuyu ended up married off to the Sashihai. In English, she explicitly blames Asahi for coming up with the idea, whereas in other languages it was merely obvious to his parents that they should should dump all resources into him and cut their dead weight (her) so that Asahi would have as much clout as possible when he inherited control of the house. (If Asahi's mother was truthful, it would be irrelevant, I suppose, since Yotsuyu's father would be likewise dead; that branch of the Naeuri wouldn't have any effect on their situation.)
So for some things we're stuck with a "wait and see".
However, the Doman-turned-Imperial in the cutscene that is patronizing the Naeuri, saying that Asahi's potential is obvious, that continued value to the Empire could mean a rare education, that it's Garlean law that all children attend school, etc. makes a very creepy sort of eye contact with Yotsuyu when her mother suggests that they will obviously continue to be good citizens.
When we see this officer outside of the Echo, he's an old man referred to as "Haunted Elder", who [A] believes that he's just done what he had to to survive [B] that Yotsuyu is truly evil [C] that it's no surprise she's truly evil after what she's been through [D] that she'll burn it all before she ever kneels again [E] that he deserves what's coming to him, but it should have been a Doman finishing him off.
I wonder if the "Haunted Elder" was another Sashihai, and secured the marriage of a cute young girl for some creepy uncle or whatever to curry favor within his clan, not realizing just how bad she'd have it, or what she'd become as a result. It'd explain quite a bit.