The twins' mother, Carmen Barahona, was charged Saturday with first-degree murder in the death of the girl, Nubia.
The father, Jorge Barahona, has pleaded not guilty to attempted first-degree murder of the boy, Victor.
Miami-Dade Police Department director James Loftus is holding a news conference Monday afternoon about the latest developments.
The girl's body was found last month in West Palm Beach, wrapped in plastic and doused with a toxic chemical in the back of the truck. Her brother was found in the front seat of the truck, critically burned by a chemical. He was recently released from the hospital.
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