An 18-year-old boy has admitted before the Ramsgate High Court to raping his two sisters and brother, KwaZulu-Natal's Mercury newspaper reported on Thursday.

The boy, from Umzinto, was upset because their father was in jail for raping his sisters and blamed them for this, the paper reported.

The boy cannot be named to protect the identity of the victims. According to the State's indictment, before Judge Achmat Jappie, the accused had decided to take revenge.

The teenager admitted to raping his 13-year-old sister on 20 January. He further admitted to holding her down seven days later while a friend raped her. He also allegedly raped his nine-year-old brother and 12-year-old sister.

The children's mother and a social worker were expected to testify on Thursday morning, the Mercury reported.