One theme in Katherine Mansfield's "A Cup of Tea" is how the aristocracy treat other people. Another theme is how the aristocracy, despite their wealth, are insecure and often project onto others the attributes they hate about themselves. In her singular concern for status and wealth, Rosemary represents the aristocracy.
The wealthy Rosemary Fells invites the poor Miss Smith home with her after she asks for enough money to buy a cup of tea. She thinks she is being charitable by inviting the poor girl home, but she really wants the chance to show off her lavish lifestyle. In order to feel good about herself and what she has, she needs others to tell her how good she has it. While chatting with Miss Smith, she considers the differences in their lives and considers herself lucky.
Rosemary's husband, Philip, is not happy to find Miss Smith in his home. He asks his wife to kick her out, but Rosemary refuses to do it, having promised to care for her. Rosemary changes her mind after Philip tells her that he finds Miss Smith attractive. Convinced that the poor girl could seduce Philip and displace her, Rosemary kicks her out. She is unwilling to sacrifice what she has for Miss Smith, and she knows that in Miss Smith's position, she'd likely do the same.