Formal:
* സഹോദരി (sahodari): This is the most general and formal way to say "sister."
Informal:
* അനിയത്തി (aniyatthi): This is used for a younger sister.
* ചേച്ചി (chechi): This is used for an older sister.
You can also use സഹോദരൻ (sahodaran) for brother.
The best word to use will depend on the specific situation.