The Question and Answer section for The Murder of Roger Ackroyd is a great resource to ask questions, find answers, and discuss the novel.
Sorry, this is only a short answer forum Your question is too detailed.
Gradesaver has an entire page devoted to the themes of this novel. Simply follow the link at the top of the page to the study guide and navigate from there.
They lived together and he confided in her. Although Dr. Sheppard was villainous, I think he did care about his sister.