A nursery school teacher and twin boys, aged four, were found dead in her burnt-out flat after the fire was started to cover up their deaths, investigators have said.
The mum, named locally as Polina Titova, had recently separated from her husband and had a new boyfriend, it was reported.
Detectives have arrested a 35-year-old man believed to be the partner, Russian media reports.
The 36-year-old, from Kirovgrad in Sverdlovsk Oblast, was found dead by firefighters while they were tackling the blaze.
The bodies of her twins, who have not been named, were also found.
All three bodies were found covered in blood, reports say.
Polina had reportedly recently broken up with her husband, who has not been named.
Neighbours heard noises on the other side of the wall on the night of their deaths, according to media outlet E1.
Three engines and 10 firefighters raced to put out the fire, according to the Ministry of Emergency Situations.
Police reportedly set up roadblocks around the city and arrested the new boyfriend as he was trying to leave town.
It is currently unclear if the suspect has been charged.
The investigation is ongoing.
