It depends on which sibling you're referring to and who the sibling is.
In japanese when referring to siblings there is a formal form, (used when speaking about other people's family) and informal (when speaking about your own.)
So it would be like,
Formal:
Onii-san (your older brother)
Otouto-san(your younger brother)
Onee-san(your older sister)
imouto-san( your younger sister)'
ex: Imouto-san wa nan sai desu ka. How old is your younger sister?
San is added to the end of the formal version to show politeness.
And for Informal:
Ani(my older brother)
Ane(my older sister)
for otouto and imouto, the suffix "-san" is dropped as the politeness is not needed.
ex: ane wa sensei desu. My older sister is a teacher.
However it is very possible to use the formal version of these terms when referring to your sibling. My recommendation for that is to use "uchi" for the first part of the sentence. This isn't necessary however. Just be sure that the listener/reader knows who you are referring to!
Uchi no Oneesan wa totemo kirei. My older sister is really beautiful!
I hope this helped :)
