Hi Maggie I agree with you if the tests prove Sheila did not write that letter this does not mean she did not murder her family then kill herself. The only way the CT can be certain is to get a Graphologist to look at the documents maybe they have done this who knows 
Everyone agrees Sheila wrote the notes, don't they ?
It's weather the notes suggest Sheila was planning to committ suicide & take other people with her.
As there has been no annoncenent in the last 8 months, It can only assumed they didn't.