Flowers in the Attic by Cleo Virginia Andrews

Flowers in the Attic is a 1979 novel by Cleo Virginia Andrews. Flowers in the Attic is the first book in the Dollanganger Series, and was followed by Petals on the Wind, If There Be Thorns, Seeds of Yesterday, and Garden of Shadows. The novel Flowers in the Attic is written in the first person from the point of view of Cathy Dollanganger. Flowers in the Attic was adapted into a film of the same name in 1987.
Narrator Cathy Dollanganger is twelve years old and the second of four children (following older brother Chris, who is fourteen years old, and preceding twins Cory and Carrie, who are both five years old). Cathy and her siblings live in Gladstone, Pennsylvania, with their father, Christopher and their mother, Corinne. Their idyllic life ends when the father dies in a highway accident on his 36th birthday.
Facing financial destitution, Corrine decides to take her children to Foxworth Hall, her family home in Charlottesville, Virginia. She writes to her mother, Olivia, persuading her to let her and the children stay in the giant mansion. Olivia agrees as long as the children are hidden; she does not want their grandfather, Malcolm, to know about them. Corinne tells the children that her parents are very rich, but were upset with her as they were strong Christians and disowned her when she married her half uncle and so she has not seen them in years. She also relates that their grandfather is dying and if Corinne can earn back his love before his death, she will be the sole heir to a vast fortune. Corinne and her children take a train to a station in the middle of nowhere and walk the remaining distance to Foxworth Hall under cover of darkness... (Summary by Wikipedia)