PLYMOUTH – Roksana Sikorski was only 15 when she plotted with her adult boyfriend to kill her family inside their Plymouth Township home in October 2014.

She got a fish fillet knife, sneaked into her 12-year-old brother’s room, put her hand over his mouth and cut deeply into his throat, seriously wounding him.

The now 17-year-old was sentenced Monday to 10-20 years in prison.

Sikorski’s parents, Laurene and Jeff, adopted her and two siblings from Poland after they’d endured and abusive early childhood. 

They say her co-defendant Michael Rivera, now 24, coerced her into the conspiracy, and they blame law enforcement for not arresting him sooner after he had illegal sex with her.

“She’s in desperate need of continued psychological help,” the mother told the court Monday before sentencing. “She is still fragile, and an adult prison would shatter her delicate world. How will sentencing her as an adult help her? Is this justice?”

Sikorski took a plea deal Feb. 22 to one count of assault with intent to murder in exchange for six other felony counts getting dropped. 

Rivera is serving a life sentence after being convicted by jury on all seven counts.

The 4-foot-8, 95 pound defendant Roksana Sikorski tearfully apologized to her family on Monday, “for not being the daughter they wished I would be. And i promise that I will get better, no matter what happens.”

Wayne County Circuit Judge James Callahan said this a case of a young person “swayed by an older person,” who had previously had victimized her with “inappropriate sexual behavior.” 

Sikorski was arrested in October 2014. Her co-defendant, Michael Rivera, now 24, was previously sentenced to life in prison. In a separate case more than two months before the knife incident, Sikorski was his victim; the two had had sex, and she was only 15 years old.  Rivera in that case pleaded guilty and was sentenced to 2 1/2 to 15 years in prison.

Prosecutors called it a “truly haunting” plan between her and Rivera to kill all four family members. Inside the home that night, she’d been receiving anonymous cell phone messages using the Kik app, with instructions on how to kill the family, including an anatomical image for guidance.

Sikorski’s parents, Laurene and Jeff Sikorski, who adopted Roksana and her two siblings from Poland more than a decade ago, have said from the start that she was manipulated by Rivera into the murderous plot.