Nana is Mirai's elder sister. This is already explained in Chapter 18 page 2.I'm so confused. So each first daughter of the lineage is a priestess? Like Nana's mom is a first born and Mitsu's mom would also be firstborn. Or can any daughter be a priestess? Isn't Nana the second daughter, so the power went to her instead of Mirai which may have earned her special treatment? But I guess if it's a ritual, you can technically do it with any daughter, regardless of whether they're firstborn or not, just like murai did to current pregnancy.
The daughters born through the cursed womb are PHYSICAL clones of the first priestess but aren't actually her. As such they share her powers but not her soul. Since they all inherited her powers it's actually safe for THEM to use the cursed womb spell since their power can mitigate the risk. The implication here is that because Mirai cheated and used the cursed womb on herself while not having any powers, she invited the first priestess into her firstborn daughter. This was somewhat OK since it just gave Kaya a second superpowered personality that's technically still a protector being - an angry one, sure, but she still protects. The problem is that Mirai then used the cursed womb spell a second time, which allowed other things (not the original priestess since she's already in Kaya) to enter her.