What is the spanish for half brother and sister?

* Half brother: *Hermano por parte de padre/madre* (brother on the father's/mother's side) or *Medio hermano* (half brother)

* Half sister: *Hermana por parte de padre/madre* (sister on the father's/mother's side) or *Media hermana* (half sister)

You can also use the more informal terms:

* Half brother: *Hermano de padre/madre* (brother of the father/mother)

* Half sister: *Hermana de padre/madre* (sister of the father/mother)

Important: The specific term you use will depend on the context and level of formality.