While the prosecution held that the case was the 'rarest of the rare' and the accused should be awarded maximum punishment, the duo claimed that they were innocent.
Nino Mathew and Anu Shanti, both software engineers working with an IT company in Technopark here, were found guilty by Principal Sessions Judge V Shersy.
Shanti's husband Lijesh, who survived the murder attempt, told reporters that the two should get the highest punishment.
According to the prosecution, Nino Mathew, the first accused, had hacked to death his paramour Shanti's daughter Swastika and mother-in-law Omana (60) and also made a vain attempt to murder Lijesh, who escaped with injuries.
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Public Prosecutor Vineet Kumar said the accused were found guilty under Section 120B (conspiracy), 302 (murder), 307 (attempt to murder), 449 (criminal trespassing), 201 (destruction of evidence) and 380 (theft) of IPC and 67A of the IT Act.
Forty nine witnesses were examined and 85 documents were used as evidence, besides 41 material evidence.
The court also took cognisance of digital evidence and proof produced by forensic science laboratory.
Nino Mathew, after committing the twin murders, had waited for nearly half an hour at the crime spot for Lijesh, who was not at home at the time of the incident.
Lijesh, who was attacked, managed to run out of the house and alerted neighbours, leading to Mathew's arrest, police said.
His statement identifying the accused was the crucial evidence in the case.
Police had seized the tools used for the murder and bloodstained clothes during a raid at Mathew's house.