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

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.