Here's a breakdown:
* nakababatang - younger
* kapatid - sibling
* na babae - female
You can also use the more informal bunso which means "youngest" and is commonly used to refer to the youngest sibling, regardless of gender. However, if you want to specify that you're talking about a younger sister, you can say bunso kong kapatid na babae.
Here are some examples:
* Ang nakababatang kapatid na babae ko ay nasa Amerika. (My younger sister is in America.)
* Siya ang bunso kong kapatid na babae. (She is my youngest sister.)
* Napakabait ng nakababatang kapatid na babae ko. (My younger sister is very kind.)
It's worth noting that Tagalog is a language with a lot of regional variations, so you might hear other ways of saying "younger sister" depending on the region.