Formal:
* Ma petite sœur: This is the most literal translation and is appropriate for formal situations.
Informal:
* Ma petite sœur: This is also common in informal situations, especially if your sister is young.
* Ma sœur cadette: This literally means "my younger sister" and emphasizes the age difference.
* Ma petite chou: This is a cute and affectionate way to refer to your sister, literally meaning "my little cabbage".
* Mon petit bout: This literally means "my little bit" and is a very affectionate way to refer to your sister.
Other options:
* Ma sœur: This is the most basic way to say "my sister" and can be used in both formal and informal situations.
* Ma petite: This is a very affectionate way to refer to your sister, but it's not as specific as "my little sis".
The best way to say "my little sis" in French depends on the context and your relationship with your sister.