Sakuya's death could've been accidental. Since, if her "murderer" was really Sadao, then Sakuya would then be connected with Hinata's old home and her hatred of cats (specifically pissing in the garden). Hence, Sakuya's death being after when Hinata (old) dies and the later generations move on and abandon the old house. Maybe Sadao took care of Sakuya (cat) when he was young, and when the house was abandoned, he told the white lie that he would be back or something, which eventually formed into resentment. The fear of water could be both a play on cats disliking swimming, since it makes their fur wet & w/e, and rain seeping into the remains of the floor of the old house.