The woman, together with her brother, subjected the 60-year-old woman to a humiliating and extended aggravated rape in her home in Haparanda in northern Sweden.
The court of appeal for Upper Norrland cut the penalty imposed by Haparanda district court which had in June sentenced the woman to six years in prison.
The court confirmed the district court conviction of the woman for aggravated rape, robbery and unlawful detention.
The 33-year-old woman and her brother paid a visit to the 60-year-old woman in January. The woman was familiar to her victim as she had previously had a relationship with her son.
The pair forced the 60-year-old woman to masturbate for her brother and also perform oral sex on him. The woman was also forced to perform oral sex on her former daughter-in-law.
The woman was then kept captive in her home during the night.
The 33-year-old woman was also instructed to pay 187,000 kronor ($29,525) in compensation to her victim.