Final answer:
Mary observed a struggle with social skills in her niece, mirroring the detrimental effects of social isolation on child development as seen in Danielle's case.
Step-by-step explanation:
From the information provided, Mary observes a struggle with social skills in her niece despite her ability to cooperate with other kids. This observation aligns with the effects of social isolation on children, as evidenced in the case of Danielle, who, because of her nearly complete isolation, lacked development in various social behaviors. Not only was Danielle's verbal communication severely impacted, leaving her unable to verbally communicate, but her socialization process was also fundamentally hindered, as she had not learned basic social norms, cultural expectations, or how to use cultural objects like a spoon or a ball.