Formal:
* സഹോദരൻ (Sahodaran): This is the most formal and widely used term for brother.
Informal:
* അണ്ണൻ (Annan): This is used for an older brother.
* തമ്മൻ (Thaman): This is used for a younger brother.
* ചേട്ടൻ (Chettan): This is also used for an older brother, but it is more casual than "Annan".
* ചേച്ചി (Chechi): This is used for a sister, but it is sometimes used playfully to address a brother as well.
The best way to choose the right word depends on the context and the relationship you have with the person you're talking to.