Garden of Shadows explains the origin of the grandmother from the first book in which V. C. Andrews series?
Garden of Shadows:  The novel explains the origin of Olivia Winfield (the grandmother from "Flowers in the Attic") and the events that cause her to become the cold, domineering mistress of Foxworth Hall and how Corinne's childhood and eventual betrayal come about. Flowers in the Attic: Flowers in the Attic is a 1979 Gothic novel by V. C. Andrews. Flowers in the Attic:  It 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".
Dollanganger Series