Point taken but possibly semantics.
They were largely ignorant of the facts pertaining to Sheila's illness and were therefore not afforded a sufficiently informed awareness, regarding the risks or potential consequences. I think this limited understanding greatly inhibited the scope of their thinking, which was already biased anyway.
I would say that 'outside the box' in this case, would require (a) some understanding or recognition that mothers could commit such crimes and (b) an informed understanding of how seriously ill Sheila herself was (including historic incidents relevant to her illness - for example her paranoid and quasi-religious ranting and raving).
I honestly don't believe that the relatives were open to exploring such avenues, which would in any event have presented an inconvenience to them. They had painted themselves in to a corner and did not want to consider inconveniences.
I believe a lot of people here are ignorant of the facts about schizophrenia - people suffering from mental illness are more likely to become the
victim of crime than the perpetrator - this case is no different. The relatives may not have known much about Sheila's illness, but Jeremy did and made good use of it.
Historical incidents did not include violence merely her own 'fears. On top of this, she was taking her medication - she had no choice because it was injected and the dose left in her systems at death was still considered to be a moderate - not low dosage!