One-year-old dies after alleged attack on Sunday, while three-year-old sibling remains in critical condition

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A baby has died in a Sydney hospital five days after a mother allegedly stabbed them and two other children in the Blue Mountains, police say.

Bystanders were first on the scene after finding the three children and the 42-year-old woman with stab wounds inside a vehicle at Valley Heights in the Blue Mountains, west of Sydney, on Sunday afternoon.