In the original show Bewitched (1964), Tabitha was the older child. In this show, she is Adam's kid sister.
Tabitha's birth was aired in January of 1966 and Adam's was in October of 1969. That would have made their ages eleven and seven, respectively, when this series premiered.
Tabitha and Adam are both in their 20s, despite the series taking place in 1977, only five years after the end of Bewitched (1964) in which they were depicted as young children.