I saw this movie around the early 80s. This journalist (?) ends up on an island thing. Maybe not an island but somewhere remote and with a gate to the outside.
Anyway, there’s a family living in this remote place and they take him in and he discovers that the mum and dad are brother and sister.
The climax is when the father uses a cannon (don’t know why he had a working cannon) to kill the family bc he thinks the visitor will dob them in.
Maybe something like that, I was pretty young and that was a long time ago.