AMSTETTEN , Austria ( CNN ) -- The Austrian who reportedly admitted holding his daughter captive for 24 years and fathering seven children with her will plead insanity , his lawyer said . Josef Fritzl admitted to authorities he raped his daughter and fathered her children . Attorney Rudolf Mayer said he believed 73-year-old Josef Fritzl had a mental disorder , The Associated Press reported . Mayer said someone who was mentally ill `` did n't choose '' to do what police allege he did . Mayer said Fritzl would be confined to a psychiatric institution rather than a prison if he was certified as insane and convicted , AP reported . Police said last week that Fritzl had confessed to imprisoning his daughter in a secret cellar dungeon in his home , fathering seven children with her and burning the body of one who died in infancy in a furnace . Meanwhile , Fritzl 's sister-in-law has said his wife did not know her daughter was held captive in their basement for decades because she had been trained not to ask questions under her husband 's tyrannical rule of the household , . `` He was such a tyrant , '' said the woman identified only as Christine R. in a Saturday interview conducted and translated by AP . `` He tolerated no dissent , '' Christine R. added . `` Listen , if I was scared myself -- I was scared of him at a family party and I did not feel confident to say anything in any form that could possibly offend him -- then you can imagine how it was for a woman who spent so many years with him . '' Watch what the sister-in-law has to say » . In the televised interview , Christine R. added detail to the bizarre story of 73-year-old Fritzl who was recently arrested and confessed to holding his daughter captive in a dungeon under the home for decades , repeatedly raping her and fathering seven kids -- with six of them surviving . Christine R. also said Fritzl committed an unrelated rape in 1967 , served 18 months in prison for that crime and that her sister continued to stay married to him in a desperate attempt to keep their family together . Recent media reports also claimed Fritzl had been convicted of rape . Austrian police have said they are looking into the claims . The horrifying story has shocked many locally and across the world . On Sunday , members of the Amstetten religious community held a Mass to remember Fritzl 's family . After the Mass , members of the church signed a banner outside a church in support of the victims . Watch church members sign the banner » . The story of the family 's imprisonment began to unravel two weeks ago , when Fritzl 's daughter , Kerstin Fritzl , fell seriously ill with convulsions and was hospitalized . The 19-year-old girl , who had been locked in the basement her entire life along with her mother and two brothers , was in an artificially induced coma in an Amstetten clinic . She was suffering from a kidney ailment that worsened because she did not receive medial treatment sooner , authorities said . Watch how Fritzl led double life » . Fritzl told his wife that their daughter Elisabeth , who is now 42 , ran away from home at age 18 . The couple adopted three of the children who Josef said were left on their doorstep as infants by his runaway daughter . In the interview Christine R. said her sister , Rosemarie , truly thought that her daughter had ran away to join a cult . `` She never believed him being capable of it , '' said Christine R. `` We were all taken in by him and believed that she ( referring to Elizabeth ) , was in a cult and that she would n't come out . '' It may have been Fritzl 's strict rule over the household that made it possible for him to keep his gruesome secret hidden for so long , Christine R. said . The unspeakable ordeal has taken a toll on the whole family , Christine R. said , stating that she spoke to her sister on the phone recently . `` Five or six days after Kerstin went to the hospital I called my sister and asked her how the girl was doing , '' Christine R. said . `` She said that she herself was doing badly , and the girl herself was doing badly and she wished with all of her heart that the girl would pull through . '' Fritzl is being held in police custody . He has yet to be charged , but he can be held by police for 14 days without formal charges while the investigation is under way . That amount of time can be extended by a judge . E-mail to a friend . Copyright 2008 CNN . All rights reserved.This material may not be published , broadcast , rewritten , or redistributed . Associated Press contributed to this report .
NEW : Lawyer of Austrian incest suspect says his client has a mental disorder . Fritzl imprisoned and raped daughter , also fathered her children , police say . Wife of Josef Fritzl was too scared to question him , her sister says . Fritzl 's wife focused on keeping family healthy , according to her sister .