Formal:
* Nakababatang kapatid na babae (This is the most literal translation, and is very formal)
* Nakababatang kapatid (This is less formal, but still indicates respect)
Informal:
* Kapatid kong babae (This literally means "my sister," but it's understood to be the younger one in this context)
* Ate (This is a term of endearment for an older sister, so you would use it for your older sister if you have one)
Note: The term "bunso" is used to refer to the youngest child in the family, regardless of gender. So, you could also say "bunso kong kapatid" to refer to your younger sister.
Examples:
* "Mayroon akong nakababatang kapatid na babae." (I have a younger sister.)
* "Ang aking nakababatang kapatid ay isang doktor." (My younger sister is a doctor.)
* "Ang kapatid kong babae ay maganda." (My younger sister is beautiful.)
* "Ang bunso kong kapatid ay nag-aaral pa." (My youngest sister is still studying.)