How do you say little brother in hawaiian?

There isn't a single word for "little brother" in Hawaiian. The closest translation would be:

* 'ōpiopio kaikuaʻana (pronounced oh-pee-oh-pee-oh ky-koo-ah-nah) - This literally means "younger brother".

You could also say:

* kaikuaʻana liʻiliʻi (pronounced ky-koo-ah-nah lee-ee-lee-ee) - This literally means "small brother".

The best way to convey the meaning of "little brother" would depend on the context and your specific relationship with the brother.