Kaya is the result of what happens if non-psychic hands delivers the baby before it has a chance to rip itself out of Womb-sama. It's a life that is almost more phantasmal and a living curse, but is also alive and far exceeds the capabilities of a priestess. The Ebisumori never take births this far normally, but the reason for Kaya's strength is because she's a failed Hiruko-sama that was taken out of the womb before the curse could be complete.
For Kaya, Mirai ate Yoshii's umbilical cord. It's possible she managed to get her own umbilical cord from when she had Kaya, citing religious reasons for keeping it, only to devour it and try again once Kaya had started smashing ghosts.
Grandma had wished for the blood sacrifice practices of the Ebisumori clan to end, so she tried to give Mirai a normal life by distancing her as much as possible from the rest of the family. Mirai was never brainwashed, but due to her alienation, her growing grudge against her own family for isolating her, and from the realization that Yoshii was used as a Womb-sama, it lall drove her beyond the point of insanity. At some point, between running away from home and marrying her husband, she had intended to use the curse to destroy the Ebisumori, but she had always intended to even when she was pregnant with Kaya-chan.
Granny is terrified of Kaya because she's likely what a Hiruko-sama looks like, to some extent. Black skin, vacant eyes, she likely thought Mirai had gone through with the ritual and the rest of the clan was already dead, which drove her insane. Grandma had caused Yoshii to die and give birth to a Hiruko-sama, and she's the only one who knew what happened to the baby and whom it was sent after to kill. The curse only triggers once the mother dies, so it's possible Granny had literally killed Yoshii in a ritual against the last of their enemies , but Yoshii was always willing and eager to sacrifice herself because she was already deeply indoctrinated into the cult. The only remaining person who can serve as Womb-sama is Mirai and Granny knows just how much Mirai hates her family, but never realized Mirai discovered the truth behind the death of Yoshii. In which case, one must ask: How could Mirai have learned about the Cursed Womb without discovering the truth behind Yoshii's death? There aren't many books written about the curse itself.
Kaya is terrified of Hiruko-sama because it's an insanely powerful curse meant to kill people EN MASSE. Kaya's soul is a partial blend between herself and Hiruko-sama, but the actual form of the Cursed Womb releases a living, violent, and vengeful god to wreak utter death and annihilation to the enemies of the Ebisumori. Kaya can recognize just how foul the Cursed Womb really is and likely can also sense the black vengeance and maligned, evil intent coming from her own mama, hence both Mirai and Hiruko-sama taking on monstrous forms to Kaya-chan. Mirai's soul is empty while the monster in her womb is gathering curse energy and writhing in misery.