There is no smoking gun, but there is a whole host of circumstantial evidence regarding Jeremy's relationship with his parents, the true nature of which he is still in denial over after 32 years, the provisions of the will, his bitter state of mind over the period as related by Julie, his proficiency in firearms, his materialistic nature and desperation to impress others by money to compensate for his inadequacy, and the combination of circumstances which accrued, including Sheila's illness, Nevill's infirmity, the lightning strike and the occasion of all the family assembled under the one roof, making as Julie reported that the crime would be "now or never".
Half of this is based on your assumptions while the other half is based on the word of a proven liar.
Now lets look at credible circumstantial evidence against Sheila.
the circumstance: of the re-admission was that Mr. Neville BAMBER ‘phoned
the hospital requesting that she be re-admitted urgently due to
deterioration of her mental state.
Sheila was found to be psychotic and this time thought that her boyfriend
Fredey was the devil.See Dr Furgusons statements
Sheila had a violent and short temper
I would say that Sheila had a very quick and violent temper.See F Ahami statements
People would not feel safe around her while having a psychotic episode.
I was in fear for my personal safety.See F Ahami statements
Sheila had mentioned killing people
"All People Are Bad And Should Be Killed" See notes of DS Jones/Barbra Wilson
Sheila was suicidal
She asked if I had ever thought of killing myself, and that she had contemplated suicide on more than one occassion
See statements of Helen Grimster
RC was once sent out to take the twins away from her because she might harm them.