In this moment, Nuttel primarily assumes that the niece is a stranger to the area, like him. This is reflected in his lack of familiarity with the local community and his reliance on the letters of introduction from his sister. The context indicates that he is unsure about the specifics of the aunt's life and circumstances, which suggests he hasn't had the opportunity to learn about the people in this setting. Therefore, the most fitting response is:
He assumes the niece is a stranger to the area, like him.