Final answer:
Marjorie doesn't like Bernice because she perceives her as a drag on her social life. Bernice's lack of social skills and inability to be vivacious or engaging causes frustration for Marjorie, who is concerned with maintaining her social image and popularity.
Step-by-step explanation:
At the beginning of the story, Marjorie doesn't like Bernice because she feels Bernice is a drag on her social life, and none of the boys wants to dance with her. Marjorie perceives Bernice as boring and socially inept, which Marjorie believes reflects poorly on her own social standing. Marjorie refers to Bernice as a lame-duck visitor, indicating that she sees Bernice as a burden. Despite efforts to help Bernice become more socially adept, Marjorie is frustrated by Bernice's lack of vivacity and inability to engage with others in a way that would bolster Marjorie's social image.
Marjorie's disdain for Bernice grows as the latter begins to attract the attention of Marjorie's beau, Warren, which further complicates their relationship. Bernice's increased popularity threatens Marjorie's social dominance, leading to jealousy and a shift in the social dynamics within their circle. Marjorie's interactions with Bernice are fueled by her perception that Bernice's social failings are a reflection of her own reputation.
Ultimately, Bernice's lack of social finesse and its impact on Marjorie's social life is the key reason for Marjorie's initial dislike towards her cousin. Marjorie's concerns revolve around popularity, appearances, and the social games played among her peers.