After nearly 11 hours of questioning, Soumya admitted to having committed the murders of her two daughters and parents, they said.
The woman had killed her two children and parents reportedly after they objected to her wayward lifestyle, they added.
Soumya was picked up for questioning after forensic reports revealed the acute presence of Aluminium Phosphide, a poisonous substance, which is commonly used in pesticides, in the body of the elderly couple.
Soumya's father Kunhikannan, 76, mother Kamala, 68, and her eight-year-old daughter Aishwarya had died in a span of three months this year.
Keerthana, one of her two daughters had died in 2012, police said.
When Souwmya's another daughter, Aishwarya died in January last, her body was allegedly buried inside the premises of their residence near Pinarayi, without post-mortem.
When the elderly couple also died under similar circumstances, relatives and locals raised suspicion and a post-mortem was conducted.
Subsequently, Aishwarya's body was exhumed yesterday by the police and the viscera was sent for chemical examination to check the presence of any residue of Aluminium Phosphide in her body.
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