Trying to find a mystery, printed (in paperback) before 1984. A small town sheriff is faced with a baffling murder. Spoiler alert! It turns out that a well-to-do brother and sister are rather more than that…and want it kept unknown. I believe the story is set in Montana or Idaho, but can’t be certain.
Stump the Bookseller
New format, same book stumper solving.
If it was published a bit later (1987) it could be The Killings At Badgers Drift by Caroline Graham. It’s the first of her Midsomer series, that the BBC Midsomer Murders were based on. It’s set in England though.https://www.fantasticfiction.com/g/caroline-graham/killings-at-badgers-drift.htm