Spoiler for ep 5:
Interesting, I was reminded of kj1980's question. So how did Mion know about Keiichi and Oishi meeting in the restaurant?
It was established at the end that Shion do exist and she works in Angel Mort, so we can struck out dual personality... for now. But the twins doesn't seems to be getting along, so I don't think we can trust that Shion passed on the information to Mion (Yeah also how did Shion know about Keiichi then). That leaves that Mion stood-in for Shion from time to time, possible since her part-time work is on and off and Angel Mort could every well be one of her uncles' business, though I can't imagine Mion willing to wear that outfit, not when we are shown how red-faced she can get. Oh yeah that sure is a cute side of her.
Other reasons? None that I can think of at the moment. But I'm sure Shion is introduced for a reason that has to do with Mion going insane. The episode and it's title also seems to be hinting that Mion is bothered by her shy side and she could be jealous of Shion's openness.
As Sushi-Y said, Shion does seem to fall into the evil scheming type. The way she took the credit for the obento and tried to seduce Keiichi is evident. Probably why Mion never introduce Shion to her friends. I'm sure everyone can see that there's more going on between Mion and Shion too. She acts just too unnatural whenever Shion is mentioned, she's clearly avoiding the topic. Maybe she is bullied or threatened by her sister?
Can anyone clarify for me the relationship between Hinamizawa, Okinomiya, and Shishibone City? Doesn't seem important really, but I thought it might help me understand the area of influence the Sonozaki family have. Thanks.