Superior Court Judge Michael Levanas issued the ruling after considering a report by child protective investigators that was critical of Winter's mother, Chrisoula Workman.
The report found there was evidence of emotional abuse toward Winter, Levanas said, and the agency planned to step in and take its own action if Winter did not remain in a guardianship.
The ruling came more than six weeks after Workman was temporarily stripped of custody amid allegations she had been physically and emotionally abusive to Winter.
Levanas said the investigators' report found that claims of physical abuse were inconclusive, but an attorney for Winter's sister said they could be proven in a subsequent report.
The judge set a Dec. 12 trial date to determine whether Winter should be placed in a permanent guardianship.
Winter has been under the care of her sister, Shanelle Gray, since early October. Levanas said that if the case remains in his courtroom, he will make sure the family works toward reconciliation.
"I guarantee if it stays here, it will get better," the judge said. He said the Department of Children and Family Services could still choose to file its own case, which would remove it from his jurisdiction.
Winter's mother has denied all accusations that she was abusive to her daughter.
Winter's father, Glenn Workman, also filed an objection to the guardianship late Monday, stating he wants a better relationship with his daughter and would be willing to care for her.
"Even the judge today agreed that reunification is so important for a family, and while Ariel is still temporarily with her sister for now, we remain hopeful she will be home soon," Chrisoula and Glenn Workman wrote in a statement released Tuesday evening.
Levanas said Glenn Workman's filing was in conflict with statements he gave to investigators and that he could not give him custody of the young actress. He also noted that Glenn Workman described his estranged wife to investigators as "controlling and driven."
"A controlling mother does not mean she's unfit," Chrisoula Workman's attorney Anita Gumm said in court Tuesday. Gumm said after the hearing that the allegations of emotional abuse would also be proven false.
Gray was also taken from Chrisoula Workman's care in the 1990s amid accusations of abuse. Chrisoula Workman contends Gray was a rebellious teen who left home and that she is contending with a similar situation with Winter, who has been acting since age 7 and stars as Alex Dunphy on the hit ABC series "Modern Family."
People on the show's set are concerned about Winter's well-being and have been sneaking her food, according to in-court statements by Gray's attorney, Michael Kretzmer.
"Not for one second is this the picture of a rebellious teen," Kretzmer said during the hearing.
Gray's filing states Chrisoula Workman has repeatedly slapped Winter and engaged in emotional abuse, including name-calling.
"The allegations made by Ariel are false," said Gumm. "We really feel she's just a rebellious teen and wants her independence. It's our hope that the court terminates the guardianship. Both parents want Ariel home. Shanelle is not suitable to be a guardian."
Guardianship cases in California are public record. Attorneys for Winter's sister requested the case and its proceedings be sealed, but a judge refused. The case was filed under Winter's birth name, Ariel Workman, in part to avoid attention.
Although The Associated Press does not generally name underage victims of abuse, Winter's name is being used because it is included in the public guardianship case.
Glenn Workman's filing Monday does not address the abuse allegations, but it states he believes his daughter should be returned to live with Chrisoula Workman, or that he be allowed to raise her.
"I want to provide for her a calm loving home environment that is a retreat from the glitz and glamour of Hollywood; a place where she can think and relax without any distractions," Glenn Workman wrote. "This whole situation has turned into a circus and places Ariel in a position she should not have to be in."
Winter's credits include appearances on several TV series, including "ER" and "Phineas and Ferb," and movies such as "Kiss Kiss Bang Bang," "Ice Age: The Meltdown" and "ParaNorman."
Anthony McCartney can be reached at http://twitter.com/mccartneyAP.