Formal:
* Ma sœur: This is the most common and direct translation.
* Votre sœur: This is used when talking about someone else's sister.
Informal:
* Mon frangine: This is a more casual and affectionate term.
* Ma petite sœur: This is a sweet and endearing way to refer to your sister.
The best choice depends on the specific situation and your relationship with your sister.