Possible chronology:
Marmelo - Hiroki and Rena have sex for the first time.
Rose - Direct continuation of Marmelo.
Statice - Hiroki and Rena still in the early stage of their relationship.
Freesia - Mana not yet in relationship with Hiroki.
Lyon - Hiroki surprised to find out that sisters know about his relationship with Rena. Mana jumps in on the fun.
Viola - Direct continuation of Lyon.
Azalea - Direct continuation of Viola.
Cattleya - Yuna in a relationship with Hiroki.
Phalaenopsis - Hiroki still in high school
Dendrobium - Hiroki gets into the university, cover suggests that foursome is already complete.