Formal:
* छोटी बहन (chhoti bahan): This is the most common and formal way to address your younger sister.
* नन्ही बहन (nanhi bahan): This means "little sister" and is also quite formal.
Informal:
* बहन (bahan): This is a more casual way to address your sister.
* छोटी (chhoti): This is a very informal and affectionate term, meaning "little one."
* बहनू (bahnu): This is a very affectionate and playful term used between siblings.
Other options:
* प्यारी बहन (pyaari bahan): This means "dear sister" and is a very affectionate term.
* मेरी छोटी (meri chhoti): This means "my little one" and is a very close and personal term.
The best term to use will depend on your relationship with your sister, the context of the conversation, and your personal preference.