I'm not sure whether I'm right but as I understood Keiichi was dating Mion in the beginning of episode. (To avoid misunderstandings: I call "Mion" the one who lives in Hinamizawa and "Shion" the one who lives in Okinomiya). So I think Keiichi met Mion in Angel Mort when first time visited it with his father, then it was Mion who gave him bento the next day, it was Mion who saved him from bikers. And he met Shion first time only when he visited Angel Mort on that special event.
Well, I know that when he first time saw sisters together after that Angel Mort event Mion seemed to know nothing about dates with Keiichi, but as I understand she just was embarrassed to admit that it was her, right? And Shion was just lying to tease her, right?
The reason I think so is because Mion/Shion told Keiichi about strife against dam project after she (Mion or Shion) rescued Keiichi from bikers, but the day before festival Shion, Tomitake and Takano started telling Keiichi about the curse, and when Keiichi said he already knew about dam war Shion said "Oh, my sister have already told you". So if Shion didn't know that Keiichi had already heard about it from one of the sisters in Angel Mort, it means the one who Keiichi visited in Angel Mort was Mion, right?
The only thing confusing me is why would Mion give Keiichi ticket to special event and then NOT swap with her sister.