The Sin that attacks Dream Zanarkand is the same as the Sin that attacked Spira. If you'll recall, Jecht became Braska's final Aeon, which Yu Yevon used as the template for a new Sin at the time of the Final Summoning. Since Yu Yevon was summoning, in essence, the very Dream(s) of the Fayth, there's no reason why Sin wouldn't have access to Dream Zanarkand. This is particularly true in light of the fact that Jecht, "Sin", was from that Dream. Jecht's reduced consciousness was, in fact, the reason Sin appeared at Dream Zanarkand in the first place.