Police have not revealed many details about the case
Police have arrested a man and his son on suspicion of raping their daughters in the northern Italian city of Turin.
The son was allegedly encouraged to imitate his father, who reportedly kept his daughter captive in a dark room for 25 years, sexually abusing her.
Italian media say the first victim, given the pseudonym "Laura", is now 34 and is having psychological treatment.
Laura alerted the police, who now believe that the 63-year-old man's son also raped his own four daughters.
Laura's abuse is believed to have started when she was nine years old.
Prosecutors say she was locked up in a room without electricity, forced to leave school at the age of 13 and never allowed to leave the house without her father's supervision.
According to Italian media reports, the case came to light after Laura accused her brother of rape.
She had reportedly fled her father's house, sought refuge in her brother's house, only to be allegedly locked up and raped by her brother for two weeks.
Laura says her brother also raped his own four young daughters.
'Droit de seigneur'
"It seems that in this family there was a sort of 'droit de seigneur' of the father over the daughter," prosecutor Pietro Fornon said, alluding to a feudal right said to have existed in medieval Europe giving the lord the right to sleep the first night with the bride of any of his vassals.
Father and son, both street vendors who collect scrap metal for a living, have been arrested and charged with rape, sexual abuse and carrying out obscene acts in public.
Both men have denied any wrongdoing.
Laura was reportedly already known to police after filing a complaint for sexual abuse 16 years ago.
But as the BBC's David Willey reports from Rome, no action was ever taken because the woman was allegedly forced by her father to pin the blame on an uncle who also denied any wrongdoing.
Investigators say the case is unusual in that most of the other members of the large family of the accused - he has eight sons and two daughters - defend their father and have told police that they worship the ground where he treads.
The younger children have now been taken in charge by the city authorities and are undergoing therapy.
Italian media say the case echoes that of Josef Fritzl, the Austrian man jailed for life on 19 March for having kept his daughter as a sex slave in a cellar for 24 years and fathered her seven children, murdering one of them by neglect.
Italian police announced the arrests on Friday.