The relatives were already wealthy.
They simply didn't believe Sheila in her fragile condition killed her mother, father, two sons, and herself. By reloading twice and brutally fighting Nevill. Without getting any gun residue, oil, injuries or damage to her nightie.
Bamber was the only other possible suspect. With a motive, opportinity, no alibi and knowledge there was something wrong at the farm.
They had a point.