Actually it could be quite simple .Sylvia didn't die on the T'uesday evening but some time in the night. (Remember the picture of her body on the mattress and how the legs stick out with rigor mortis?). The events Shirley told of as of Tuesday morning and later about Sylvia giving "a death rattle" did happen but they shifted the time. Could not have been difficult with a child so young. So they either witnessed Sylvia dying or found she had died, down in the basement. Gertrude was screaming and hollering with sheer panic, the shovel noise was Gertrude trying to bury Sylvias body indeed, against all logic and agaainst better knowledge, in the concrete floor (o rmaybe she tried to loosen some bricks?) and the basement was "all tore up" the next morning (as Marie said) because the kids were trying to get hold of Gertrude and stop her. Could it be?