Final Answer:
Mrs. Stapleton didn't want her brother to overhear her warning because she suspected her brother, who was actually her husband, of being involved in the sinister occurrences on the moor. Her caution was to prevent him from knowing that she was suspicious of his actions.
Step-by-step explanation:
Mrs. Stapleton's reluctance to have her brother, actually her husband, overhear her warning stemmed from her suspicion about his involvement in the mysterious events occurring on the moor. Her actions were a careful maneuver to conceal her distrust and avoid alerting him to her suspicions. Knowing that her husband was orchestrating the malevolent incidents, she needed to shield her thoughts from him to prevent any potential danger to herself or Dr. Watson.
In "The Hound of the Baskervilles" by Sir Arthur Conan Doyle, Mrs. Stapleton's clandestine warning to Dr. Watson reflects her awareness of her husband's deceptive nature and sinister intentions. Her caution not only serves to protect herself but also aids Watson in navigating the treacherous situation without alerting the perpetrator. This subtle exchange between Mrs. Stapleton and Watson hints at the complex dynamics and the deceptive nature of characters involved in the mystery, adding depth and suspense to the narrative as the plot unravels.