Dimitri Belikov and his sisters are the products of a royal moroi and a dhampir having a kid and あなた don't see them being considered royal. Royal is exclusively for the moroi. Dhampirs are the products of interbreeding moroi with humans and eventually dhampirs with moroi, so I'm guessing that some moroi think of them as being unpure. They aren't even vampires, and ヴァンパイア always hold royal positions.
