Mixon faced a judge on Thursday for a drug charge from October. She was arrested two days ago and the case is unrelated to the weekend shootings.
Saying "I don't understand," Enjoli Mixon appeared confused when she appeared at the hearing before Judge Willie Lott.
Last October, the 24-year-old was arrested on a misdemeanor charge of being under the influence of drugs.
Authorities say she skipped her pre-trial hearing.
"She failed to appear in court on October 23, 2008, so the judge did issue a bench warrant. She is in custody today because she was picked up on that warrant," said District Attorney Rebecca Warren.
Mixon is being held at Santa Rita County jail in Dublin.
Sources tell ABC7, police have interviewed her to find out what she knew about her brother Lovelle Mixon, the parolee who killed four Oakland police officers on Saturday.
Mixon fled to her apartment after shooting two officers nearby on MacArthur Blvd. There, he killed two members of the SWAT team before other officers shot him to death.
ABC7 spoke to Enjoli Mixon earlier this week and at that time, she denied knowing anything about her brother's guns. She also said she had no idea if the weapons were hidden in her home.
"No, no. I don't, with the little one, no I don't," said Mixon earlier this week.
At the end of the hearing, Mixon slammed the door as she was led into the holding area -- crying and yelling.
Mixon's friends and family were not in court. The judge continued her case and Mixon will be in court on Friday morning, just hours before the police officers' funeral.