Four other family members accused of taking part in the ritual, including the female victim's now 16-year-old son, were handed suspended sentences ranging from 18 months to two years, national news agency DPA reported.
Prosecutors said the victim, 41, named in court as Seonhwa P, was set upon after she began talking to herself and lashing out during a hotel stay in December 2015, leading the accused to attempt the "exorcism".
The woman's chest and stomach were beaten while she was gagged first with a towel and then a cloth-covered coat hanger during a two-hour ordeal, they said.
The victim suffered "massive chest compression and trauma to the neck" and died of asphyxiation.
Judges at the district court in the western city of Frankfurt found the victim's 44-year-old cousin, described as the main perpetrator, guilty of inflicting bodily harm causing death.
The guilty woman, named as Doean K., had "taken it upon herself to decide over life and death", prosecutor Nadja Boettinger said during the trial.
Also accused were her son, 22, and daughter, 19, as well as a 16-year-old cousin.
In delivering his ruling, the presiding judge said the family had acted in the belief that they were chasing away the victim's demons, calling it "a tragic case", according to DPA.
After finding the woman's body at the hotel, police at the time also discovered a possible second victim in the garage of a house the group had rented in the town of Sulzbach.
The second woman, who was found alive, was badly injured and suffering from hypothermia and dehydration.