Do you think Sheila would write letters to her birth mother and simply carry them with her for weeks or months, Jane?
I suspect she wrote them after shooting June and the boys, and while Nevill was lying against the Aga, thats why she was already talking about a terrible mess, because it had already happened, or at least she was in the midst of the shooting with three down one to go!
Given that she and Christine had bonded so well and, allegedly, there were plans for Sheila and the boys to visit her in Canada, along with Colin's claim that he'd not seen Sheila as happy since she'd given birth to the twins, I find it unlikely, in the extreme, that she wrote them. It's highly likely, given how they'd always gone above and beyond for her, that June and Nevill may have offered to fund the trip -of course, that's entirely speculative- and such wouldn't have been something JB would have been happy with, on several levels.