The incestuous relationship between somewhat identical twins Haruka Kasugano and Sora Kasugano happened during their childhood, when Sora returned home after being kept in the hospital for a long time and Haruka fell in love with her at first sight. He even kissed her during a childhood game, which left Sora enamoured of Haruka ever since. At first, he didn't want that kind of relationship with his sister and denied his feelings for the next three years. He further tried to push his feelings aside by entering in a relationship with Nao. However, after Sora confessed her love for him and kissed him, he gave into his feelings for her and they proceeded to make love, beginning a relationship. When Kozue and Nao visited the Kasugano twins, they caught them making forbidden love and Kozue says although she loved him, she will never see him again and Nao ends their relationship on the spot. Ashamed of his actions, he thought of breaking off his relationship with Sora. Sora got so upset she attempted to drown herself in the lake that is located behind the shrine but Haruka chased her down and almost resulted in both of them drowning but Sora rescued him. After the near-death experience between them, they give in to their feelings and profess their love for each other and decided to move to a foreign country to find happiness as a couple.
In a manga sequel/fan disk, it is revealed that Haruka and Sora moved to Scandinavia to live with a family friend to find happiness together.I do not know about manga, but in fan disk's Sora's route they do not moved to live with a family friend, that was just a trip, this friend invited Sora and Haruka and even sent them tickets. And that was not Scandinavia, but Estonia. Inscription on the ring is in Finnish language, yes, but Sora speak with the seller on Estonian language. So ring is just import (or do You think that in Finland Estonians sells rings? Well, maybe-maybe. But than, how does Sora found out, that seller was Estonian? If he work in Finland, he speak with his clients on Finnish, yes?).