Jeremiah Oliver, 5, was not reported missing until December, although relatives say they had not seen him since September. Police have said they are investigating the disappearance as a homicide case.
His mother was arrested in December and referred for a mental health evaluation. She is being held in lieu of $100,000 bail, while bail for Sierra is $250,000.
James Reardon Jr., Oliver's court-appointed lawyer, said after the hearing that he does not think she is capable of assisting in her defense. He said she is "scattered and inconsistent" when he tries to talk to her and cannot understand more than a few sentences at a time.
During the hearing, Oliver said she wanted another lawyer or to represent herself. A probation officer told the judge before she was brought in that Oliver had refused to sign a form to be used to determine if she is eligible for court-appointed counsel.
The judge ruled Reardon would remain as her lawyer.