* Titi: This translates to "little sister." While it doesn't specifically denote a baby, it would be understood in context.
* Teina: This means "younger sibling" and could be used for either a younger brother or sister.
You could also use a phrase like:
* Teina wahine: Literally means "younger woman" or "younger female" and could be used to refer to a younger sister, but it doesn't necessarily imply a baby.
Ultimately, the best way to convey "baby sister" would depend on the specific context and the age of the sister. You could even use a descriptive phrase like "te tamahine iti" (little girl) to make it clear.