Artemis and Apollo had a younger brother.
However, he didn't want to become an Olympian, bickering and fighting with his family all the time.
Because he didn't become an Olympian, people slowly forgot about him, eventually fading him. The only people who remembered him were Artemis, Apollo, Leto, Zeus, and his love (not telling you who she is).
The Doors of Death clattered open a few centimeters, and then shut again, but that tiny gap was enough for one soul to escape- Aaetae*, Artemis' and Apollo's younger brother.
*(AA-e-taae)