You're correct. I can't possibly 'know', as none of us can ever 'know' what was the real relationship between the two sisters. However, there are clues in what she said to June and what June said to her, and perhaps more in what she/they didn't say. I'm not going off at wild tangents, creating impossible scenarios, many of which are just opportunities to say the family were all liars.
It was accepted, at least momentarily, that Sheila had been the culprit. So why would Pam have failed to mention an earlier phone call in which June expressed fears about Sheila's mental state. Had such occurred, it could only have served to support the belief in her culpability.
I'm no expert, and I knew that when you said Pam and June were sisters.
So there was blood shared between then, well, who knows what secrets would be hidden behind the thickness of blood.
"So why would Pam have failed to mention an earlier phone call in which June expressed fears about Sheila's mental state." Well that would take June to first see the fragility of 'Sheila's mental state'.
From the little bit I know about June is that she was a bit over the top, in her ways. Was it her way or the no-way highway?
This song expresses what I was trying to say:
https://youtu.be/dG5xv6h-mpw"Gabriela Gun?íková - MY WAY OR THE HIGHWAY - Original Song by Ken Tamplin"