Formal:
* 家姐 (gaa3 ze2): This is the most formal way to say "older sister."
* 細妹 (sai3 mui6): This is the most formal way to say "younger sister."
Informal:
* 姐姐 (gei2 gei2): This is a more casual way to say "older sister."
* 妹妹 (mui6 mui6): This is a more casual way to say "younger sister."
* 阿姐 (aa3 ze2): This is a very informal way to say "older sister." It's often used by younger siblings or people close to the older sister.
* 阿妹 (aa3 mui6): This is a very informal way to say "younger sister." It's often used by older siblings or people close to the younger sister.
Other options:
* 妹 (mui6): This is a shorter, more general way to say "sister." It can be used in both formal and informal contexts.
* 姊妹 (zi2 mui6): This is a more literary way to say "sister" and is used in formal situations. It can also refer to "sisters" in a general sense.
The best way to choose the right word depends on your relationship with the person you're talking to and the situation.