The girls have been hospitalized in serious condition. They are 7 and 9 years old.
Their aunt, Lisa Turkki of East Syracuse, was arraigned on two counts of assault and is being held at a county jail. The name of her attorney was not on record early Monday.
The Journal News says she was watching the girls while the parents attended a concert down the street.
Katonah is a hamlet of Bedford. It's about 35 miles north of midtown Manhattan.