"Ma sœur s'appelle..."
Here's a breakdown:
* Ma: My (feminine singular)
* sœur: sister
* s'appelle: is called (formal, used for names)
You would then follow with your sister's name. For example:
* "Ma sœur s'appelle Marie." (My sister is called Marie.)
Other possibilities:
* "Le nom de ma sœur est..." (My sister's name is...) - This is a bit more formal and less common.
* "Ma sœur, c'est..." (My sister, it's...) - This is more informal and used for friends or close acquaintances.