Who insisted on riding the sled with Buck, along with her husband and brother?

The correct answer is that Mercedes insisted on riding the sled with Buck, along with her husband and brother. In "The Call of the Wild," Mercedes is characterized as somewhat inept and spoiled, representing the human element that is disconnected from the harsh realities of the wilderness. Her insistence on riding reveals her lack of understanding of the difficulties inherent in the journey and her view of the sled as a place of comfort rather than a means of survival. This ultimately reflects the broader themes in the novel of the clash between civilization and the primal instincts necessary for survival in the wild, demonstrating how her actions contribute to the struggles faced by Buck and the rest of the sled team.

The other characters mentioned do not fit this specific scenario as Mercedes is the only one who insists on such comfort during a time that calls for endurance and resilience.