Why is the Third Avenue apartment a source of conflict between Sarah and Iris in the novel, The Verifiers?

Sarah's third avenue apartment is in a very nice neighborhood and is worth millions of dollars. Her father leaves it to her after he passes away, but she cannot afford to keep up with its bills since she is mostly unemployed, so Iris pays for the upkeep. The apartment is a big source of tension between the two sisters, because Iris sees it as a sign that her father loved Sarah more than he loved her.