From Creation Entertainment:

"Although Rob, Kristen and Taylor will not be with us this time out we will have a super line-up of other Twilight stars and the weekend will be filled with special events, movie prop and costume displays, contests, panels, auctions, parties, exclusive merchandise, music and much more, including some surprises!"

From THR:

But according to Pattinson's manager Nicholas Frenkel, the truth is that "We have not canceled any confirmed professional appearances for Robert. Specifically, he was never invited or set to attend these events." A source close to Stewart concurs, adding "This is just another event that she definitely didn't cancel because she was never scheduled to attend in the first place."