Im Kaydens wife's feed

Please give me some spoiler, From what I gather, it seems Sellonia's father is the root of all this disaster, but what I don't understand is, if that's not Sellonia's soul, then where is the real Sellonia's soul? And if the real Grace is in the basement, then who is the fake Grace? And if Grace has a younger sister, why was her younger sister's and father's face never shown from the beginning of the story? So, couldn't Grace's real family recognize the change in their own child? Or have her father and younger sister also been brainwashed by that fake Grace? Please, someone, give me an answer to my question if you know.