The sisters had been shopping at pharmacy in Hamilton.
They returned to their car to find the suspect waiting for them inside.
Police say the 92- and 94-year-old women scared the man off by screaming and hitting him.
"He has a knife," Hamilton Township police Sergeant Robert Miktus said. "He's reaching toward the front seat with the knife towards the women. The driver, who's 94 years old, she threw her arm back to block him, to block the knife. In the meantime, the other sister, the 92-year-old sister in the passenger seat, jumped from the car, waved her arms and started screaming, attracting attention."
Eyewitnesses saw the man run away, and police arrested him a short time later.
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