Holly Staker had a sexual experience three years before she was raped and murdered in a Waukegan apartment, her twin sister testified Wednesday.
The revelation by Heather Staker, now 28, came after a heated debate between prosecutors and defense attorneys who sought to bar her testimony.
It could prove to be a significant development in the third trial of Juan Rivera, as prosecutors seek to explain how DNA that did not come from Rivera was found inside Holly Staker's body after her death.
Rivera, 36, has twice been convicted and sentenced to life in prison for the Aug. 17, 1992 slaying of the 11-year-old girl but new trials have been ordered after each conviction.
His defense team is arguing because DNA from a different man was found, it is impossible for Rivera to be her killer.
Prosecutors maintain it is possible some other man had sex with the girl in the days before her death, despite her age.
A DNA expert previously testified the semen he examined was degraded, and could have been up to four days old.
Police say in his confession, Rivera admitted having sex with the girl, but claimed he did not remember if he left any semen because he was high on drugs.
Heather Staker testified she and her sister had their first sexual encounter when they were 8 years old.
"We were both curious about the same things at the same time," Heather said. "We shared everything, even underwear."
Heather Staker testified in the previous trials, but Wednesday was the first time she spoke about any sexual activity on the part of her sister.
Before the testimony, Rivera's attorneys sought to block Heather Staker from testifying about any of her sister's sexual activity that she did not witness or have other firsthand knowledge of.
Defense attorney Jeffrey Urdangen has frequently called the idea that an 11-year-old girl was sexually active "absurd," and demanded prosecutors outline what Heather Staker was going to say before the jury was brought into the room.
But Assistant State's Attorney Michael Mermel argued the testimony was admissible because it supported their theory of the events.
Evidence of prior sexual activity by Holly Staker, Mermel said, coupled with the expert's statement about the DNA could be presented to allow the jury to consider there could be a reasonable explanation for the fact DNA other than Rivera's was found.
Urdangen warned Circuit Judge Christopher Starck the testimony risked "injecting error of the greatest magnitude into the case," but Starck allowed Heather Staker to testify without restriction.
Under cross-examination by Urdangen, Heather Staker denied a defense team report that she had asked to meet with Rivera in 2006. She said she was told Rivera had asked for a meeting with her, and she rejected the request.