Complex sentence: Cora had not come to the funeral at all-it was Miss Gilchrist in disguise, as part of a plot for her own gain, leaving Cora home asleep from a sedative in her tea.
Split into simpler sentences: Cora had not come to the funeral at all. It was Miss Gilchrist in disguise, as part of a plot for her own gain, leaving Cora home asleep from a sedative in her tea.