@pinkdaizygirl: Your concerns about Cass, Stephanie, and Jason are valid in wanting them on the show. But I wouldn't expect anything, this is an alternate universe from what is in the comics, so histories and events will be different. And any relationship in this show will be canon but only canon for this show...Again since it doesn't apply to the comics.
Young justice isn't a direct adaptation of new earth, it's an entirely different universe. Its possible that Jason and/or Cassandra don't exist on this earth.
DC Entertainment has said the Young Justice series takes place on Earth-16. This was done because it was largely untapped and freed the series from established continuity restraints set by the main DC Universe or previously seen other Earths in the DC Multiverse.