This question already has an answer here:
In Harry Potter, it is understood that the act of Self Sacrifice is a means of protecting one or more people from death from Avada Kedavra.
Sacrificial Protection happens at least twice in the Harry Potter series:
- Lily sacrificing herself to attempt to save Harry.
- Harry sacrificing himself to attempt to save the defenders of Hogwarts.
But then we have a scene that Harry sees by looking into Voldemort's mind. Voldemort is looking for Gregorovitch and murders a german-speaking family who lives in Gregorovitch's old home:
... He raised the wand. She screamed. Two young children came running into the hall. She tried to shield them with her arms. There was a flash of green light - (Deathly Hallows, Ch. 12 - Magic is Might)
This event is similar to the scene when Voldemort kills the Potter family in the sense that the mother attempts to protect the children by using her body as a shield.
Why does this act of sacrificial protection not have the same result as in the other instances?