The quantum of sentence will be pronounced later.
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.
According to prosecution, Nino Mathew, the first accused, had hacked to death his paramour Anu Shanti's daughter Swastika and mother-in-law Omana (60) and also made a vain attempt to murder her husband Lijesh, who escaped with injuries.
Forty nine witnesses were examined and 85 documents were used as evidence, besides 41 material evidence.
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Prosecution said they felt it was the 'rarest of rare' cases and maximum punishment should be given to the accused.
The court also took cognisance of digital evidence and proof produced by forensic science laboratory.
Video clippings seized from the laptop of the accused were also accepted as evidence by the court.
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.