A woman allegedly assisted her husband while raping her 20-year-old sister at their house in Willowmore, Eastern Cape police said on Saturday.

"The young woman had visited her sister who asked her to sleep over at her house," said Captain Sandisile Nelani.

The woman's brother-in-law allegedly went into the visitor's bedroom while she was sleeping at around midnight and tried to rape her.

The woman's 25-year-old sister was woken up by her screams as she called out for help.

"The sister went to the room and helped over power her sister, while her 40-year-old husband repeatedly raped her until 2am on.