Renesmee is actually a species called a dhampir.

Definition of DHAMPIR: A Dhampir (also dhampire, dhamphir oder dhampyr) in Balkan folklore is the child of a vampire father and a human mother. In vampire fiction the term Dunpeal is also used, most often the result of transliteration from Japanese manga. Dhampirs often have powers similar to a vampire, but none of their weaknesses (though, the reverse can occur as well).[1] A dhampir is believed to be unusually adept at killing and detecting vampires