Formal:
* Dada mdogo: This is the most literal translation, meaning "younger sister."
* Mdogo wangu: This means "my younger one" and can be used for both siblings and other younger family members.
Informal:
* Dogo: This is a more casual term for "little sister" and is often used among friends and family.
* Dada: While this usually means "sister," it can also be used to refer to a little sister if the context is clear.
The best way to choose the right term depends on who you're speaking to and the relationship you have with them.