You have two siblings who feel a need to return to the old house they grew up in, where they were raised by their intelligent, powerful, and twisted grandfather. Years ago, the house partially burned down, destroying both the grandfather and the sadistic, overweight 'offspring'. The younger of the two siblings, Peggy, has huge gaps in her memory that she can't fill while her brother, Eric, remembers but won't tell.
Used availability for J N Williamson's Evil Offspring
April 1987 : USA Mass Market Paperback