Anyone ever confuse about how to use these words? As a translator myself, sometimes I also need judgment to decide which one to use.

If you use google translate, the translation of “brother” is “saudara laki-laki” and “sister” is “saudara perempuan”. These are correct. However they are very rarely use, and even considered stiff. In fact, the word “saudara” is more common to use when you refer to your extended family, like your cousin, uncle, aunt, etc.

When talking about brother and/or sister, natives usually call them “adik” if younger or “kakak” if older, regardless of the gender.

So how do you know if it is female or male? Well you don’t, unless you ask, or unless they say “kakak perempuan” / “kakak laki-laki” etc.