They offered to take care of the 2-year-old son of their family friend only to drive the boy to death by depriving him of fluids as punishment for bedwetting.
The brother and sister duo appeared in court in Connecticut Wednesday to face charges over the child's death.
Police said 23-month-old Amari Jackson died of dehydration while Sharon Patterson and her brother Robert Patterson were caring for him in February.
Investigators said glasses around the house were laced with hot sauce to keep the boy from drinking.
Sharon Patterson told police she had been drinking and smoking marijuana the day before Amari died, according to an arrest warrant affidavit. She also told them she had previously lost custody of four children because of drug addiction. Attorneys said in court Wednesday that she had mental health issues.
Sharon Patterson also said she was frustrated that the boy's mother was not coming by and checking on Amari or bringing him any clothes or food.
"She was planning on depriving Amari Jackson of liquids for a week and didn't realise that Amari could get sick by dehydration," the document said
The boy was pronounced dead at Yale-New Haven Hospital, where medical records indicate that he "appeared wasted" and had "sunken eyes."
Sharon Patterson, 39, was charged with manslaughter, cruelty to persons and risk of injury to a minor. Her 31-year-old brother was charged with cruelty to persons and risk of injury to a minor.
The pair did not enter pleas during their brief appearance in Meriden Superior Court. Judge Arthur Hadden increased Sharon Patterson's bond from $US250,000 ($A269,980) to $US500,000 calling the allegations "shocking in nature."
He left Robert Patterson's bond at $US150,000.
Amari's mother, Sara Hicks, said she had been friends with Sharon Patterson's daughter since they were eight. She said Sharon Patterson offered to care for her son for about a week because she was sick with a fever and caring for her eight-month-old daughter.
Hicks said she could not make sense of her son's death but was pleased police had charged the Pattersons, news agency AP reports.