Horror Film That’s An Ever Distant Memory

This is a tough one, even my amateur director brother-in-law who watches a new movie each day doesn’t know what it is.

It’s a distant memory, I probably saw this 40 or so years ago when I was about 10 or 12. My folks were out, elder brother was taking care but was in his room with his girl.

Anyway, the film was likely black and white and the chief protagonists were a teenage girl and her younger brother.
The girl was blonde and I think she had murdered her parents in bed whilst they slept. There was a brief image of the father covered in blood (so it may have been in colour or just my imagination).
I remember a scene on a boat after that, which is where my memories end.

Any help gratefully accepted.

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