Read manga: Momoko Manual

manga
Summary:

Momoko is a girl who was born into a witch family in which most of its female members possess magics. Yet they can only reveal their magical capability to "the one" i.e. their soul mate/spouse. The more magic she uses in front of strangers, the more parts of her body would become cat-ish (such as cat ears and whiskers), and if any stranger learns her true identity she would turn into a cat completely--like her mother!