The accused, identified as Kiran Makhwana (37), was in a live-in relationship with Sonali Chavan (also in her 30s) who had a 14-year-old son from an earlier marriage.
Kiran along with his two children from his wife used to live with Chavan and her son Kunal in Nallasopara township of Vasai in Palghar, Tulinj police station inspector Bhajirao Kalantre said.
However, of late there used to be frequent quarrels between the accused and the woman as he apparently felt that she was paying more attention to her son and neglecting his children, he said.
At around 3 AM, Kiran himself walked into the police station and informed about the offence following which police arrested him, Kalantre said.
The accused has been booked under section 302 (murder) of the IPC, he said adding that further probe is on into the incident.
The two bodies have been sent to a hospital for postmortem, he said.