How does Judith discover that Teddy and Natalie were switched at birth in the novel, The Two-Family House?

When Mimi marries a wealthy man in an elaborate wedding, someone points out that Mimi seems to fit in with her husband's family better than she fits in with her own. The comment causes Judith to think about other members of her family, which is why she is able to piece together the argument between Rose and Helen to discover that the women switched Teddy and Natalie at birth.