A scan from Shonen Jump was posted a few minutes ago on FFAC Reunion. It shows Serah in both a CGI render and ingame screenshots. The scan also reveals that she is indeed a L’cie and has a mark on her left arm. She is a very nice girl who tries to solve her own problems without getting others involved. The scan confirms that she is indeed Lightning’s little sister:
View the scan here
Furthermore it is indeed confirmed that Serah is the fiance of Snow. The pendant she wears was a present from Snow and has an engagement ring attached to it. The pendant will also be sold by Square Enix Products. It resembles the logo of Final Fantasy XIII, which once again emphasizes that Serah will be a very important character in Final Fantasy XIII.
When the parents of Lightning and Serah died, Lightning started to call herself “Lightning” as a code name. With her new name in mind, she thought she could take care of Serah as an adult. She doesn’t like the relationship between Snow and Serah and according to the past trailers this will lead to some huge problems…
Some of the information above is gathered from various Final Fantasy XIII fansites and should be taken with a grain, spoon or box of salt until Square Enix confirms this with an update for the official Final Fantasy XIII site.