Guillen was a Houston-native soldier, training at Fort Hood before being murdered in 2020.
Cecily Aguilar waived her right to a trial by pleading guilty to a single count of accessory after the fact and three counts of making a false statement.
She faces 30 years in prison with a $1 million fine and an additional 12-year supervised leave after her sentence.
Aguilar is the only person charged in Guillen's death after helping her then-boyfriend, Army Specialist Aaron Robinson, hide Guillen's body.
"I'm very taken back by (Aguilar's) decision (Tuesday)," Mayra Guillen, Vanessa's sister, said. "There's still a lot of mixed emotions. Both angry, and frustration. Now, we have to wait for the actual sentencing. I'm glad that we're going to have the opportunity to speak in front of the judge, and in front of her. Of course, and hear what she has to say."
Mayra would later tweet more direct and harsher language regarding the woman who was instrumental in her sister's murder.
"I hate you with all my heart Cecily. You have no idea how much you've hurt us, along with all the others involved that the public will know one day. All of your names and faces," Mayra tweeted.
This development comes months after Guillen's family filed a $35 million lawsuit against the U.S. government on the basis of sexual harassment, abuse, assault, rape, sodomy, and wrongful death.
According to court documents, Aguilar reportedly told Texas Rangers that Guillen saw photos on Robinson's phone of Aguilar, and Robinson was worried Vanessa would report him for violating the Army's fraternization policies.
Aguilar reportedly helped lead law enforcement to Robinson, before he was confronted and killed himself.
Complete coverage: Vanessa Guillen
WATCH: Suspect in Vanessa Guillen case had 'no remorse' in court, attorney says
Suspect in Vanessa Guillen case had 'no remorse' in court: attorney
Stream now: Vanessa Guillen: Remember Her Name
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