Death Loses To Child

Riddle: In a house, there lives a family of 4. There is a father, a mother, an older sister, and a younger sister. One day, the father catches a disease. A week later, Death comes to take the father's soul. The mother begs for 5 more years with her husband, and Death agrees. 5 years later, Death comes back. The older sister begs for 3 more years with her father, and Death agrees. 3 years later, Death comes back. The father, mother, and older sister turn to the younger sister for help, and she is holding a candle with a flame on the end of the wick. The younger sister says, "I have a deal to make, Death. My father will live until this candle burns out." And Death agrees. However, Death never returns. Why not?