When Ares reveals himself to Diana in Wonder Woman, just after she finds out that her sword is powerless against him, he says "Oh my dear child".
Then, Diana tells him:
before she kills him.
Why did she say that?
Because in the DC mythology, they share Zeus as a father.
Originally the comic book character of Diana Prince / Wonder Woman was 'created out of clay and brought to life' but that was changed in later comics to Zeus being her father.
Ares is a son of Zeus in both DC and Greek mythology.
So, although their age differs massively, they are half-sibling.