* Older brother: ભાઈ (bhai)
* Younger brother: ભાઈ (bhai)
While the word is the same for both, the context usually makes it clear who you're referring to. You can also add "મોટો (moto)" for older brother and "નાનો (nano)" for younger brother to be even more specific:
* મોટો ભાઈ (moto bhai): Older brother
* નાનો ભાઈ (nano bhai): Younger brother