Yurin Yurin has posted more information about their upcoming Visual Novel. mainly the new images for Runa, Takako, Shizuku, Eris and Rena is now available, featuring their new clothes.
also, updated character relation chart is posted.
http://petalsgarden.axypb.net/2013/05/27/yurin-yurin-official-website-4/
now, here is by speculation now.
chart shows that Risa and Rikka are childhood friends, this explains why Rikka calles Risa Risa-nee. indicating that they were close before they got estranged. (side note, the chart also tells that Miya is jelouse of Rikka, stating taht Rikka is scared of her and Miya is jealous of her.)
now, only part of the story that hasn’t been explained is involvement of Shizuku and Eris in this VN. it is known that from spoilers Runa and Takako may not have their own storyline, they may have some influence. however, we don’t know anything about Shizuku and Eris apart from that they will be part of the VN as well.