DETROIT (WWJ/AP) — A 16-year-old suburban Detroit girl faces up to 20 years in prison in a plot to kill her family members.

Roksana Sikorski was charged as an adult and pleaded guilty Monday to assault with intent to murder.

READ MORE: Michigan Environment Department Issues Violation Notices To Tribar In Huron River Chemical Release

The girl’s adult boyfriend is serving life for his role in the 2014 plot. Michael Rivera was sentenced in August for attempted murder and conspiracy to commit murder.

Prosecutors say the then-15-year-old girl stabbed her 12-year-old brother in the neck and tried to stab her younger sister in their Plymouth Township home. Police say Rivera guided the girl from outside the house, advising her on what knife to use.

Michael Rivera (Booking Photo)

Michael Rivera (Booking Photo)

Other messages included a diagram of the neck, instructions to cut it “like a tomato,” and to make sure the victims stopped breathing, according to reports.

The boy’s screams alerted family members, who interrupted the stabbings as Rivera and the girl fled. The boy was hospitalized, but survived the attack.

READ MORE: Community Responds After More Drownings Happen In South Haven

Investigators believe the pair plotted the attack as part of a plan to run away together. The girl’s parents have said she and Rivera, whom she met on Facebook, were involved in a sexual relationship.

Court documents show Rivera also is charged with third-degree criminal sexual conduct involving a person age 13 through 15.

Sikorski was 4-years-old when she, along with her sibling, was adopted from Poland where her lawyer says she suffered abuse.

Attorney Leslie Posner said the girl suffers from RAD or reactive attachment disorder, a rare but serious condition that occurs in children who have been neglected and are unable to form a healthy emotional attachment with their caregivers.

Sikorski will be sentenced on March 28.

MORE NEWS: Dive Team Recovers Body Of Drowning Victim From Lake St. Clair

TM and © Copyright 2016 CBS Radio Inc. and its relevant subsidiaries. CBS RADIO and EYE Logo TM and Copyright 2016 CBS Broadcasting Inc. Used under license. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report.