Accident by Design — E.C.R. Lorac

Templedean Place in the Cotswold Hills of England was among the last of the truly aristocratic estates, where old family traditions still ï¬,ourished. When Gerald Vanstead arrived from Australia with his family, to attend his father in his last illness, other, more deadly things flourished.
Gerald's wife was the bickering kind; he drank too much, was given to feuding with the chauffeur, and seemed excessively tightlipped and disagreeable -- and so no one was particularly sorry when one day the brakes on Gerald's car failed to hold, and he and his wife were killed.

A family picnic ended in the accidental death of another Vanstead, a fire destroyed what might have been a clue, and there was a night of horrible suspense before Inspector Macdonald could say who hated Gerald Vanstead the most and who, in a house of cultured, well-bred men and women, was most capable of murder.