Actually, Jim said that he didn't like the idea that she was their daughter since he thought it made it feel like MLP was too small, and that he would rather leave it up to the fans. He also said that he felt it was a plot hole with Luster not knowing that Twilight was still friends with the others, but if they only meet once a month, she could still assume that, so it's not really a plot hole. So that's a maybe/up to fans from Jim and a yes from Josh. Seems pretty canon to me.