In the 1960s, writer Bob Haney was writing The Brave and the Bold. He used the title to team up various DC superheroes, like The Atom and the Metal Men or Aquaman and Hawkman.
In any event, in mid-1964, he teamed up the sidekicks of three major superheroes in The Brave and the Bold #54, which starred Robin, Kid Flash and Aqualad.
The pairing was quite popular, so exactly a year later, Haney reintroduced the team in The Brave and the Bold #60, only this time known as the Teen Titans.
However, in the late 50s, writer Robert Kanigher, in the pages of Wonder Woman, had decided to give Wonder Woman the same approach that Superman was given, by telling tales of when Wonder Woman was a toddler (Wonder Tot) and a young girl (Wonder Girl).
These stories proved to be quite popular (so popular that, by 1965, there would be issues where Wonder Girl's name would be larger than Wonder Woman's on the title of the comic), so Kanigher's next step was, in the early 60s, to tell "impossible tales" where there would be a team-up of Wonder Woman, herself as a toddler, herself as a girl, and her mother.
Like this issue, for instance...
Or this one (gotta love how Wonder Tot spoke)...
Or, finally, this one (notice how it stresses that this pairing is IMPOSSIBLE)...
Well, Bob Haney must have casually glanced at one of these issues (which were coming out at the same time he was writing The Brave and the Bold) and when he decided to make a team of sidekicks, he figured that this Wonder Girl was Wonder Woman's sidekick, so he added her to the Teen Titans in #60.