Jac Davies died following a blaze at his family home in Alltwen, Neath Port Talbot, in the early hours of July 27.
His mother Jenny, 28, escaped the fire with her 11-month-old baby and daughter Kelsey, six, while other son Riley, three, was rescued by a firefighter.
An inquest into Jac's death was adjourned last month.
South Wales Police have confirmed a woman was arrested and bailed last month in connection with the incident.
Police have not named the woman involved but it is understood to be Jac's mother.
A spokesman said: "A 28-year-old woman from the Pontardawe area has been arrested on suspicion of child neglect and is currently on police bail while enquiries continue."
A pre-inquest into Jac's death will be held on January 25.
The mother of a four-year-old boy who died in a house fire has been arrested on suspicion of child neglect.