Usually, "kapatid" will do. However, that only means "sibling". "Ate" officially refers to the eldest female sibling, but there are times when any older female sibling is also loosely called "ate" for convenience and exact ranking is not important. That's because to describe the exact position of that sibling in the set would require the use of more words.
If you want to be clear that you meant a sister and not just a sibling, you can say "kapatid na babae (a sibling that is female)".
I just recently found out that I have a sister and she is Filipino. = Nalaman ko kail
án lang na may kapat
íd na bab
ae pal
á ako at siy
á ay Pilip
ina.
Nalaman ko kailan lang = I just learned/found out recently
na may kapatid na babae pal
á ako = that I have a sister ("Pal
á" is a word we use to express an unexpected realization of something. Without "pala", it might give the impression that you are not surprised at all to find out that you have a Filipino sibling.)
at siya ay Pilipina = and she is Filipino. (You may also use "Pilipino" instead of "Pilipina" since "kapatid na babae" already indicates the gender, but we normally use "Pilipina" to mean a "Filipino female".)