Angel Station by Walter Jon Williams (1989)
Ubu has four arms, a fantastically faithful memory and is 13 years old. His 11-year-old sister Maria is more conventionally constructed, yet so psychic she considers herself a witch. (Both mature with the artificial aid of "mones" to the time of their late teens.) Their father, Pasco, a small-time trader with an ancient spaceship, originally assembled them from spare genetic material and personality programs, but he commits suicide while in the throes of a depression, leaving his children to fend for themselves. Ubu compounds their problems by making an unwise business move, putting them deeply in debt--in danger of losing the ship and facing a life of virtual indenture. An attempt to capitalize on Maria's psychic abilities in a casino ends in disaster, leading to their arrest, escape and lives pursued on the edge of the law.