The relatives could have taken a huge risk and -
Hoped Sheila received a contact shot.
Hoped the rifle was too long for Sheila with the silencer.
Hoped the rifle nozzle had no blood on.
Hoped they could convincingly put blood into a silencer creating the back splatter effect.
Hoped there were no crime scene photos of the aga.
Hoped the blood they put in would be a match to Sheila's.
Hoped there was no other evidence that showed Sheila was the killer.
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It was a 1 in a million shot.
If they gambled & amazingly got lucky with all of this, it could mean a successful fabrication of one piece of evidence.
If the 99.9% chance of being unsuccessful happened as expected, they could go to prison.
Not at all. It was not until after Jeremy stated that he left the rifle in WHF without a silencer attached. Was the silencer then "discovered". By putting Sheila's blood in the silencer that was kept in a cupboard, it could then be convincingly argued that Sheila could not have shot herself. And Jeremy's recollection then appears untruthful.
Had the conspirators known the following -
1. Nicholas Caffell suffered two contact wounds.
2. Crime scene photos show no debris on the carpet under the cooker.
3. Contact wounds leave abrasion patterns of the muzzle.
4. Neville had three burns on his back caused by the barrel without a silencer.
5. A .22 rifle is least likely to produce backspatter.
6. Backspatter is most commonly caused by gunshot wounds to the head.
They probably wound not have dared try. The only thing they got lucky about was Rivlin QC deciding to follow a defence strategy with the silencer being attached on the gun.