It was the second tragedy of the week to raise concerns over the well-being of rural Chinese children.
The boy told police in Hunan province that the girls, aged 8 and 14, drank cola that he had laced with poison, Xinhua said. The girls died on Wednesday.
Xinhua said the suspect was familiar with the victims, but the news agency did not identify him.
On Tuesday, a 13-year-old boy and his three younger sisters died after they drank pesticide in a village in the southwestern province of Guizhou, according to state media reports and local government statements. The siblings had been living on their own, with their father working in a faraway city and their mother abandoning the family for a while.