The novel begins with Silas Lapham being interviewed for a newspaper profile, during which he explains his financial success in the mineral paint business. The Lapham family is somewhat self-conscious in their sudden rise on the social ladder and often fumble in their attempts at following etiquette norms. They decide to build a new home in the fashionable Back Bay neighborhood, and Lapham spares no expense ensuring it is at the height of fashion.
Tom Corey, a young man from a well-respected high-class family, shows an interest in the Lapham girls; Mr. and Mrs. Lapham assume he is attracted to Irene, the beautiful younger daughter. Corey joins the Lapham's paint business in an attempt to find his place in the world, rather than rely on the savings of his father, Bromfield Corey. When Tom Corey begins calling on the Laphams regularly, everyone assumes his interest in Irene has grown, and Irene takes a fancy to him. Corey, however, astounds both families by revealing his love for Penelope, the elder, more plain-looking, but more intelligent daughter who possesses an unusual sense of humor, a sophisticated literary passion, and a sensible but inquiring mind. Though Penelope has feelings for Tom Corey, she is held back by the romantic conventions of the era, not wanting to act on her love for fear of betraying her sister.
Silas Lapham's former business partner Milton K. Rogers reappears in his life, asking for money for a series of schemes. Mrs. Lapham urges her husband to support the man, whom he had pushed out of the paint company in what was deemed an inappropriate manner. Lapham's dealings with Rogers, however, result in a substantial financial loss. His major asset, the new home on Beacon Street, burns down before its completion. The Laphams are humbly forced to move to their ancestral home in the countryside, where the mineral paint was first developed.

Question: Why does Penelope fear becoming romantically involved with Tom?
Answer: Because her sister Irene has developed a romantic interest in Tom. 
Alternative answer: She does not want to hurt her sister