Willow and Piper were born in Part 17: Twins . They were mainly cared for by either Rose or their Nanny, as Don was busy with work and Amanda would be out clubbing. LifeSimmer stated that the Child-aged Willow and Piper had a special bond, therefore they would often be in each other's bedrooms.
Willow was a better student than her sister and often had to do Piper's homework to prevent her transferal to military school.
The twins Aged Up off-camera before Part 29: Weathernaught 57X.