The Question and Answer section for The Hound of the Baskervilles is a great resource to ask questions, find answers, and discuss the novel.
Mr. Stapleton was furious. Miss. Stapleton had no time to answer, as her brother stormed up and confronted Sir Henry.
With that I offered in as many words to marry her, but before she could answer, down came this brother of hers,...
In Chapter Twelve, Holmes exposes the fact that Miss Stapleton is not Mr. Stapleton's sister..... she is his wife.