Marmalade Boy (2001)

Koishikawa Miki, a high school student, is told by her parents that they are getting a divorce and swapping partners with another couple. The other couple have a son called Yu, whom Miki becomes attracted to after moving in together.

~~ Adapted from the manga series "Mamaredo Boi" by Yoshizumi Wataru

University student, Takatsuki Minato, likes things to be clean and in order while her stepbrother, Takatsuki Toru, is a bit of a player. One day, while cosplaying as a high-school student in a fashionable district in Tokyo, Minato accidentally bumps into Toru and quickly lies about her identity. To her surprise, Toru appears to take an interest in her alternate identity, Mina, and asks her out. Thinking that she might be able to cure Toru out of his playboy ways, Minato continues to disguise herself as Mina but will she be able to keep up the charade when she starts developing feelings for Toru?

A stepbrother and stepsister fall in love even though they are not blood related, their love is still forbidden.

The forbidden love between fake siblings (brother/sister) who like each other.

A forbidden love between a stepsister and stepbrother who love each other.

A forbidden love between a stepsister and stepbrother who love each other.

A forbidden love between a stepsister and stepbrother who love each other.

A stepbrother and stepsister fall in love even though they are not blood related, their love is still forbidden.

"Newlyweds" Keiichi and Akira Hirose have moved to a new city. Keiichi, who works as an author, and Akira, who works with kindergartners, are hiding a big secret from the people around them.

~~ Adapted from the manga series "Saraba, Yoki Hi" by Akaneda Yuki

In both dramas, the protagonists think they are half brothers and one of them interrupts their story.