Why does Beth question her relationship with Chris after learning her sister is engaged?

Chris has lived with Beth for many years, but Beth gets the sense that he will never propose. With her younger sister getting married at the end of the year, Beth feels the stagnant nature of her relationship with Chris deeply. Over the course of the novel, Beth struggles with the simultaneous comfort and coldness she gets from Chris. At times, Beth rationalizes his laziness, but there is an underlying frustration she has when she admits their relationship is going nowhere.