Formal:
* Akka: This is the most common and respectful term for older sister.
* Chelli: This is the most common and respectful term for younger sister.
Informal:
* Anna: This is a more informal term for older sister, used among friends and family.
* Chelli: This can also be used informally for younger sister.
Other terms:
* Veedhi: This is a more affectionate term for younger sister, used by older siblings.
* Nanna: This term is used to address a younger sister, meaning "my sister."
The specific term you use will depend on your relationship with your sister and the situation.