Harvinder Kaur was murdered at her Bhawanigarh residence here in Punjab on December 31 last year.
The police claimed to have recovered a heavy tractor equipment with which Kaur was hit on the head, leading to her death.
Investigations revealed that Harvinder Kaur's daughter- in-law Beant Kaur along with her paramour Kuldip Singh, a pharmacy student, murdered her as they were caught by Harvinder in a compromising position, police said.
After the murder, Beant Kaur went to the Bhawanigarh police station and lodged an FIR that her mother-in-law was murdered in the wee hours by some unidentified persons while she was away for morning walk, they said.
During questioning, Beant Kaur confessed that she used to mix sleeping pills in Harvinder Kaur's milk so that she could call Kuldip over while Harvinder was in deep sleep, police said.
Police has recovered gold ornaments, sleeping pills and mobile phones from the accused after their arrest under Section 302 (murder).