A friend of the mother, who has been named locally as Carly Ann Harris, 37, said they believed the youngster may have drowned in the bath.
South Wales police attended the three-bedroom housing association-owned family home in Trealaw in the Rhondda Valley at 10.20pm before closing the main route through the former mining village.
Officers confirmed that a 37-year-old woman had been arrested and taken to Merthyr Tydfil police station for questioning.
Parademics and the fire service were also at the scene, with one local saying they initially thought it was a house fire after noticing a “burning smell”.
Shocked neighbour Jody Maher wrote on Facebook how she went to the home after the girl’s death. She said: “My heart is literally aching. I never thought I would have to witness something so heartbreaking.
“Sleep tight, princess, you deserved the world and 10 times over. I just wish this was a nightmare. I wish there was something that I could have done.”
Another family friend, Amy Jayne, said: “Waking up yesterday to her playing in our garden, being her happy chatty self, to now waking up to nothing but police teams and forensics.”
Many people said that the mother doted on Amelia-Brooke, who has two older brothers.
Chief Superintendent Dorian Lloyd said: “A young child was declared deceased at the property and, at this time, the death is unexplained.
“We are continuing to investigate the events that led to the tragic death in Trealaw and an incident room has been established.
“Specially trained officers committed to providing support to the family and local community at this very difficult time, have been deployed. I am aware of the deep concern and upset locally but would urge all members of the community to refrain from speculating.
“I encourage everyone to think carefully about the consequences of messages when using social media. Positive action will be taken against those who post messages that are abusive, threatening or insulting.”