This would change the whole stance of it having been Sheila,,who people have firmly argued against it having been her. They could well all be right-----------------but it wasn't Jeremy either.
Poor Sheila had given us all the clues,,,but it could be the one that you would least expect it to be,,,,,although had also succumbed to mental illness. Mental illness in all its forms are unpredictable.
I suspect that a myriad of past and present differences came into the conversation on the night of the murders and some bad things being spoken which led to this " family dispute ".