Formal:
* छोटी बहन (chhoti bahan): This is the most common and formal way to say "younger sister".
Informal:
* छोटी (chhoti): This is a shorter and more informal way to refer to your younger sister.
* बहन (bahan): If it's clear from the context that you are referring to your younger sister, you can simply say "sister".
Other options:
* नन्ही बहन (nanhi bahan): This means "little sister" and is often used as a term of endearment.
* लिटिल सिस्टर (little sister): This is an English phrase that is sometimes used in Hindi.
The best way to say "younger sister" in Hindi will depend on the specific situation.