Yotsuyu's adoptive mother stammers and giggles through most of what she says even in Japanese, so her story isn't necessarily any more credible than her claim that she's too sickly to attend school. However, the inter-language references we do to family repeatedly specify that she calls her adoptive family by their adoptive roles. After the adoption, this woman was "mother". I'd originally suspected that "mom" was even lying about that, but it doesn't seem to be the case right now, since 4.2 doubled down on, "By Blood: Asasi = Cousin".
Still, I feel that this can probably be cleared up by focusing on two specific lines.
When Gosetsu tries to anger Yotsuyu, he does it by calling her:
Dead parents from the Naeuri clan.Originally Posted by Gosetsu
Dead husband from the Sashihai clan.
When Yotsuyu is explaining herself, she says
As far as I can tell, she's not talking about reality, she's talking about how she feels that she was seen.Originally Posted by Yotsuyu
Notice that it's increasingly severe stages of dehumanization.
They didn't see me as part of their family.
Never mind family, they didn't see me as part of their culture.
Never mind culture, they didn't even see me as a human being.
Never mind human being, they might as well have not even seen me as a living thing.
Not kin. Not country. Not human. Not living.
In Japanese the line is merely a reference to filial piety robbing her of her personhood.
"Honor thy parents. Submit to thy husband. I was no longer my own. No one in Doma considered my heart to even exist."
This part is pure speculation but I can't help but wonder if the Haunted Elder was a Sashihei himself, that he took advantage of the family's eagerness to prove themselves to the empire to secure their cute kid for some creepy uncle or something, furthering his own status in the clan without thinking of or realizing just what he was setting her up for and how she would revenge herself on their entire culture for it.
Still, between Gosetsu, 4.2's labels, and the ability to not take one of the confounding lines literally, the number of possible interpretations (at least for what the game wants us to believe) dwindles.