Looking for a book I vaguely remember. From what I can remember, there’s siblings who take a taxi or something to follow their father. Their father goes to some sort of mental institution or a hospital. And the siblings sneak in and find some shocking information. ( I wanna say it has to do with their mother. Maybe she was checked in there, but I know it’s not any of the books in the Tillerman cycle by Cynthia voigt)