Originally, the 1950s Katy was 21 years old and Sis was about seven years old. In subsequent series, Katy is roughly the same age, but Sis was reimagined as being around 15 and has been so ever since.
In the original series, Katy was 21 years old, while Sis was approximately seven years old.  Subsequent iterations of the show maintained Katy's age, but Sis was redesigned to be around 15 years old and has remained that way.