Thanks for the chapter!
Some of us has a discussion about this last chapter, but for those who've missed it, this is most likely an unfortunate incident from miscommunication with a "bit" of lack of trust and vigilant. HOLY essay incoming~
Back in Mutsu's (Kaya's grandma) generation I think we only have her and Yoshi, in which the tradition is still going strong, Mutsu getting education to become the head while Yoshi become the cursed womb bearer, Mutsu herself noticed how fucked up it was but she couldn't stop the adults from brainwashing Yoshi (since she herself was also a child)
Moving time forward, Mutsu became the head and decided to get rid of the tradition for good, starting with giving birth to Nana prematurely so her power get weaken and not letting Mirai become a cursed womb bearer by not telling her anything related to the household, at this time I expect there're those who opposed and Mutsu herself have to deal with them hence she left Nana and Mirai with Yoshi most of the time in their childhood, Yoshi most likely was the last resort to cut them all off (maybe at this point we have the technology to get her pregnant from IVF instead of getting it from a male in a branch family).
Now we get to the scene of Mirai being bullied because of her house's "infamy" and she still didn't know anything, Yoshi died giving birth to the curse, then Mutsu and Nana still refused to tell her anything further making her feeling even worse, granted it's because they didn't want to drag her into the house's affair yet that made her feel like she's not one of them or like a family, which feel unfair in Mirai's perspective because they always keep her in the dark as if they don't trust or expect her of anything.
She just happened to find Yoshi's diary on childbirth because her class needed it that one time, which then found out about the cursed womb thingy, she started hating the Ebi house itself because she thought Mutsu killed Yoshi for the sake of it. Then she ran, got a husband who actually loves her and tried to get a cursed womb, but then gave birth by accident when she was so close to finishing the curse, which is how we have little Kaya here.
Mirai then going for a retry with the second cursed baby, which was growing by the day, and Kaya can see it clearly which is why she's so scared, she can't kill the curse since the womb is in her mother's belly, and it tried to attack Kaya everytime they meet as well I assume.