"Rajaiah, his wife and son have been arrested and produced at the residence of a magistrate," Assistant Commissioner of Police (Hanamkonda Sub-Division) Shoban Kumar told PTI.
The charred bodies of S Sarika and her three children Abhinav (7), and twins Ayaan and Shriyaan (3) were recovered on Wednesday from the house of the former MP at Hanamkonda.
Following a complaint by parents of S Sarika, the deceased daughter-in-law of Rajaiah, a case was registered against the former parliamentarian, his wife Madhavi and son Anil Kumar at Subedari police station under sections 498A (husband or relative of husband of a woman subjecting her to cruelty) and 306 (abetment to suicide) of IPC, and under section 174 (suspicious death) of CrPC for the death of three boys.
The trio who were taken away by police from their house for questionning last evening were quizzed again on Thursday and were arrested based on the evidence gathered by police.
The ACP said the doctors conducted postmortem of S Sarika and Abhinav and Ayaan and Shriyaan (3) at a government hospital today.
"The report has not been given yet and the FSL report is also awaited," the senior police officer said.
The bodies were subsequently handed over to Sarika's mother Lalitha and other family members who performed the last rites.
Warangal City Police Commissioner G Sudheer Babu had said that Rajaiah, Madhavi and Anil Kumar were among those present inside their house when the incident happened on the first floor of the two-storey building.
Meanwhile, a forensic expert Dr Naga Mohan, who conducted the postmortem along with other doctors, said the side bones of Sarika and the legs of Abhinav and Ayaan were found broken.
"Thick particles of smoke were found in the chests of the dead persons," he said, adding that as the bodies were totally charred the viscera and other parts were sent to FSL for further assessment.
Meanwhile, Sarika's mother and sister on Thursday reiterated their charge that Sarika's in-laws were responsible for death
of their daughter and grandsons and that it was "intentionally" done by the former MP and his family.
Sarika and Anil, both classmates had fallen in love during their engineering degree course, and got married in 2002.
Sarika, who quit her job as a software engineer in a multinational company, had earlier alleged that her husband was having an extra-marital affair, and had even staged a sit-in protest outside Rajaiah's house.
In April 2014, a criminal case had been filed against Rajaiah, Madhavi, Anil Kumar and another woman at the Begumpet women's police station here following a complaint lodged by Sarika, alleging that her husband and in-laws were harassing her.