A close relative's role was being probed in connection with the case, West Godavari SP Raghurami Reddy said adding that they do not suspect it to be an accident.
The house of the victims at Kovvadalanka Annavaram village in Bhimvaram town of West Godavari was found locked from outside and some bottles of petrol were found there, Reddy said.
The deceased are Prakash (57), his wife Rajamani (48), and their children Devaraj (19) and Vamsi (14).
Ravi and his wife Divya are presently in Bhimavaram on a holiday and police suspect Ravi's role into the incident, Reddy said.
A case has been registered and the matter is under investigation, police said.
The SP visited the spot this morning with other police officials, police said, adding the bodies were sent for postmortem to a government hospital in Bhimavaram.