A graphic novel about a boy whos younger sister (I think), goes missing or dies and out of greif, the mother makes the town flood. Also there's 3 witches?

This is driving me insane- I remember very little about the plot unfortunately, bit i know the boy has a friend (girl) who tries to help him find out what happened to the sister, and that's when they encountered the 3 witches. His dad is also a cop, who's trying to find out what happened. The cover was the boy and the girl riding across the water in fog I believe.