A 12-year-old girl was critically injured in the incident. The mishap took place in Kokarwadi village, 120-km from here Saturday evening, said investigating officer S.M. Raut.
Since there were no petrol stations for hundreds of miles nearby, a motorcyclist went to the Kadam family, known to store petrol in a mini tank in their home.
"As there was a power failure, 12-year old girl Sonu Kadam lit an oil lamp to get the petrol in a plastic bottle for the customer. Suddenly, the petrol tank exploded in a blast, throwing her, and the customer Sitaram Kadam several metres away," Raut told IANS.
Two other members of the family, including her 60-year-old grandmother Indira Kadam and her six-year-old brother Mahadev were killed along with Sitaram Kadam.
Sonu, who sustained severe burns, is battling for life in a hospital in the nearby Sholapur district, Raut said.
Meanwhile, police are now combing the village and surrounding areas to detect families who have stored illicit stocks of petrol, diesel, kerosene and other inflammable items.
Osmanabad is situated in south-central Maharashtra, around 400-km from Mumbai.