Why does miss costello reject daisy as a suitable companion for her nephew?

Miss Costello rejects Daisy as a suitable companion for her nephew because she perceives Daisy as lacking the social status and refinement that she deems necessary for someone associated with her family. She is concerned about maintaining her family's reputation and believes that Daisy's background does not align with their expectations. Additionally, Miss Costello's protective instincts for her nephew lead her to prioritize social propriety over personal connection.