The sister of one of the teenagers charged in the deadly 2020 house fire in Green Valley Ranch has pleaded guilty to federal drug trafficking and weapon possession charges, according to court documents filed Thursday.

Tanya Bui, 24, entered a plea for one count of fentanyl possession with intent to distribute and one count of possessing a gun in furtherance of a drug trafficking crime. She has also agreed to forfeit about $6,825 in cash, a pistol and ammunition, according to court documents.

Investigators said they uncovered evidence of Bui’s drug and gun dealing while investigating her brother, Kevin, as a potential suspect in setting a house fire on Aug. 5, 2020, that killed five members of a family. They discovered social media messages discussing sales, and text messages exchanged with Kevin show she used him for helping her with drug deals, the plea document says.

Law enforcement found fentanyl pills and marijuana, a gun and ammunition, and cash when they searched the Buis’ family home in Littleton in January 2021 in connection with the arson. Tanya Bui stipulated she intended to distribute less than 50 kilograms of marijuana and more than 40 grams of fentanyl, according to the plea agreement.

She will be sentenced June 10.

Kevin Bui and two other teenagers face first-degree murder charges in connection with the arson. In January, a Denver District Court judge ruled Kevin Bui and one of his co-defendants, Gavin Seymour — both 16 at the time of the fire — should be tried as adults despite their defense attorney’s attempts to transfer their cases to juvenile court. The third teenager was 15 at the time and his case remains in juvenile court.