Sabrina Zunich changed her plea to guilty on Thursday to aggravated murder in the death of her foster mother Lisa Knoefel, Lake County Court of Common Pleas Clerk of Courts said.
Zunich, 19, was arrested immediately following Lisa Knoefel's November 2012 murder and has been in the Lake County Jail ever since.
She faces life in prison.
Lisa Knoefel's husband Kevin Knoefel was convicted by jury last month of plotting to kill his wife after having sex with Zunich, the couple's foster daughter.
Knoefel, 43, of Willoughby Hills, was convicted of conspiracy to commit aggravated murder and three counts of complicity to commit aggravated murder.
In addition, Knoefel was convicted of six counts of sexual battery involving Zunich.
Prosecutor John O'Donnell told a Lake County jury that Knoefel even instructed Zunich how to stab 41-year-old Lisa Knoefel to cause the most internal damage.
According to the indictment, Knoefel was charged with engaging in intercourse with Zunich on the day of the murder, in addition to occasions before and after his wife's murder.
Prosecutors believe Knoefel planned the murder beginning in August 2012 along with Zunich.