Atsuro and Yuuka and step brother/sister, with Yuuka--the girl--the older one. Atsuro is the friendly and cheerful one in the whole series, by the way.

So, as the story starts to unfold, he reveals gravely to her that whatever woman he's been with, he's pretended it was her. Along with this, he states that he's been in love with her for a while. This frightens and disturbs Yuuka and she flees to a friend's house.

Atsuro sees this as a firm rejection and the band begins to notice his distanced and solemn demeanor. A few days later, as Atsuro is getting ready for another concert, Yuuka slips into his dressing room. Before she can explain why she is there, he is quick to talk, saying: "I can take care of you...! I could support you and we could be together..." She simply tells him that she'd rather be with him, as she unconsciously felt all along.

Later on in the series, they try to keep it a secret from everyone, but near the end, their friends told them that they had figured it out. Atsuro and Yuuka, then, show their relationship openly, though a bit wary of telling their parents still.

The lead boy character constantly puts his sister down, ever since the beginning when they met each other. At that time, she had bounced right up to him and introduced herself, “I look forward to us being brother and sister!” In return, he gave her a nasty remark. This later is explained that he hadn’t liked the idea of both of them being brother and sister from the start.

On many occasions, he has tried to steer her away from a certain boy who likes his sister (and she likes the boy, as well). As the manga goes on, the brother finds a girl, though he doesn’t like her. At some point, they get mad at each other and she exclaims: “We should have never been siblings! This was a mistake!” and runs off. Later, they resolve the issue by sitting in their kiddy pool like they used to do as children. She apologizes to him, saying: “You know…I’m glad were siblings,” meaning she’s content they became good friends and could confide in each every once in a while. He becomes solemn and seriously tells her, “I’m not… I’m not glad were siblings.” Their friends interrupt before she can ask him to elaborate.

Mio, the main girl lead, has a brother (Hinata or “Hina”) and a mother. Her father had died when they were at a young age. As a child, Mio had played with a boy her age that was next door, named Prince. Now Prince very much liked Mio’s mom, he not having one himself. One day, they switched clothes (when he said they were going to pretend to be each other) and he planned on going as Mio to her house. But, coincidentally, an abductor had planned taking Prince for ransom (his family is very, very rich) and took Mio instead. The trauma made Mio forget who Prince was.

Later, in her teen years, it’s evident Hinata, her brother, is attracted to her, though she’s completely oblivious to it. Hinata is working for a pimp to earn money for their sick mother, though it’s revealed that Prince’s family has been supporting her for a while for seemingly unknown reasons.

Prince proposes a job for Mio and she reluctantly agrees (by now, she still doesn’t know she has forgotten him and sees him as a jerk). Prince is obviously attracted to her and constantly tries coming on to her, much to Mio’s annoyance.

Hinata and Prince never saw eye to eye, so later on in the series, they start getting along, and Mio doesn’t know why. She gets jealous that her own brother won’t turn to her for advice. One day, she pulls him aside and demands him to tell her why he’s been so distant towards her. He reluctantly tells her that he has a job at a hotel (which is only half true) because he wants to pay for school for the both of them. She feels bad and thanks him, saying she’s grateful. He explains that he had told Prince this and Prince was humbly keeping it a secret.

Later on, Hinata seems to be attracted to Prince and seduces him, causing Mio to be both disgusted and disturbed. She feels wrong talking to her own brother about the homosexual acts, so she goes to Prince. He merely brushes her off. Hinata tries explaining it to his sister, but she avoids him, saying, “It’s just weird! You’re both guys!” But she notes the heartbroken look on his face and regrets saying it. When he is able to talk to her about it again, he states that he’s never been attracted to Prince. In fact, it’s all been an act. “Why?” she asks, and he explains that it was to keep Prince away from Mio and because he loves her. She is disturbed and runs away. Prince finds her and takes her back to the house; Mio finds a note that says that Hinata has left.

It’s revealed that Mio is actually the daughter of Prince’s father, making Prince and Mio siblings. Prince is angered but still tries to pursue a relationship with her, though she is disgusted. Mio gets a hold of Hinata, who has been living with his boss, and she decides to live with him.

The other twist, is that it isn’t Mio whose the sibling of Prince, it’s really Hinata, making the two brothers.

To me, the incest was between the brothers.

the main character is in love with her older non-blood-related brother. Meanwhile, he falls in love with a girl from Australia, they sleep together & find out soon after that theyre actually half-siblings...talk about an "oh sh!t" moment.