Origin of the cannibals

That is all the explanation the author gives in the book, doubt they will offer much more in the movie (I'm watching it now). *SPOILERS* (at least for the books) As I recall, it is basically what you described: a remote lighthouse in the 1850's, during a severe storm or something with no hope of rescue until conditions clear, everyone in the family is suspected dead except the pre-teen daughter who is found weeks or months later, starving and completely traumatized. She tells the rescuers that her little sister is also still alive, out there in the woods somewhere alone. They think she's just traumatized and talking crazy so they don't look for the child. Years later, next lighthouse caretaker's family has some similar trauma, end up with a missing son. It is implied that maybe the original child had survived and ended up stealing the little boy to have some company out in the woods. The cannibals descend from this pair. Kill lots of fishermen and tourists over the years but it's always assumed that it was boating accidents and people who were not prepared to be out in the wilderness...until the events of the first book (Off Season) take place. Much eating of human flesh and general torture transpires.