Omg I read a manhwa that had a similar plot to what u said, it’s called imperfect sisters but idk the other name so u might need to search it up but the parents adopted a daughter to make their very sickly biological daughter feel less lonely until the mother found out she could transfer the sickness to the adopted daughter through a sick disgusting ritual and yea u can see where it goes its really good but it gets a lil cray cray cuz yk its the mothers fault with her selfish greed the father is also in on it but he doesn’t play a major role other than helping the mother

Hmm is the white egret related to the child? The child has a surgery on one of the arms and the egret was missing sth in their wings?