The incident came to light when Saravanan, son of Krishnaveni (70), one of the victims, came to the house early this morning, they said.
Krishnaveni and her elder sister Ranganayaki (80) were found murdered with their throats slit, police said.
On receipt of information, police reached the spot and conducted a preliminary enquiry.
Later, Ranganayagi's adopted son Balakrishnan was taken for questioning.
During interrogation he reportedly confessed to the crime leading to his arrest, police said.
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