A dishwasher tied up the hands and legs of her two young nephews in the kitchen of her Teck Whye flat.
She left them food and asked a male friend then staying at the flat to feed them while she was at work.
On July 3, 2011, two days after tying them up, the 48-year-old realised that the younger boy was motionless.
He was pronounced dead an hour later, a district court heard on Friday.
The maternal aunt had been taking care of the two boys, then aged seven and five, since March 17 when they went to stay with her.
Their parents had been arrested and remanded for investigation by the Central Narcotics Bureau.
The accused cannot be named as there is a gag order on anything that will lead to the identification of the victims.
She has pleaded guilty to tying both boys' hands to a metal beam and PVC pipe, as well as their legs with pieces of cloth, at her flat between July 1 and July 3, 2011.
She also admitted to using a wooden back scratcher to hit the younger boy's calf and buttocks on July 2, causing him unnecessary physical injury.
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