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.

AC London were formed in 2012 by Prince Choudary to help stop teenagers in Croydon falling into a life of gangs.
They entertain Crawley Down Gatwick in the extra preliminary round on Saturday (15:00 BST), with Choudary set to become the youngest manager in the competition's history.
"In 2011 we didn't exist, now we're in the FA Cup," said the 21-year-old.
As well as founding and managing AC London, trainee solicitor Choudary is also chairman of the 10th-tier Combined Counties League Division One club.
Choudary was at school when rioting took place across London in the summer of 2011.
Croydon, where Choudary was born and raised, was one of the areas badly hit.
"They broke into my uncle's clothes shop, causing him to sell up his livelihood. They went past our house but luckily nothing happened," Choudary told BBC Sport.
"The problem is the youth are misled in today's society and go down the wrong path; they choose the street life.
"It was soon after the riots that I had the idea to start a new football club. There was little in terms of activities in Croydon and I wanted to make a change."
AC London has grown rapidly since being founded as a youth team in July 2012.
Within three years they were elected into the football pyramid for the first time and spent 2015-16 in the Kent Invicta League before being transferred to the Combined Counties.
"Setting up a club from scratch is an extremely hard task to achieve," added Choudary, who has FA Level 1, FA Level 2 and Uefa B coaching badges.
"I worked part time at a local solicitor's office as a cleaner to get the extra cash."
The current squad is made up of 16 nationalities, including players from Venezuela, Brazil, Cameroon, Spain, Bulgaria, Finland, Greece and Nigeria.
AC London groundshare at Whyteleafe's 2,000-capacity Church Road ground, close to Croydon.
James Phillips is believed to be the youngest boss in FA Cup history after he led Romsey Town in the competition at the age of 22 in 2014.
Choudary was not born when Everton beat Manchester United in the 1995 FA Cup final at Wembley.
"Being the youngest manager and chairman has taken years of sacrifice and hard work that I invested to make this a reality," added Choudary, who played as a goalkeeper for the club but now concentrates on his off-field roles.
"The one advantage about being chairman and manager is that I can't sack myself."
AC London's squad includes club captain and defender Ernest Simon, who is more than 10 years older than Choudary.
Choudary said he hoped for a crowd of about 400, which would be a club record, for the visit of Crawley Down Gatwick, who play one tier higher in the Southern Combination League.
It is just 68 days since Arsenal beat Chelsea to win the 2016-17 FA Cup at Wembley.
The extra preliminary round features 370 teams who will have to win six games if they are to advance to the first round in November. Winning clubs at this stage will receive £1,500.
The BBC will stream one match per round through qualifying.
The all-Merseyside tie between Litherland REMYCA and AFC Liverpool (12:30 BST) will be live on Saturday on the BBC Sport website.
Litherland, who play in the 10th-tier North West Counties League Division One, are another side making their FA Cup debut. AFC Liverpool play one league above them in the North West Counties Premier Division.
The mother of a four-year-old boy who died in a house fire has been arrested on suspicion of child neglect.
[NEXT_CONCEPT]
A club formed by a 16-year-old after the 2011 London riots will compete in the FA Cup for the first time.