Marmalade Boy(1994)

Original title: ママレード・ボーイ

Miki's parents come home from their vacation to announce that they are getting a divorce. But what is more shocking is that they have switched partners with another couple! And to top things off, this other couple has a handsome son, Yuu, who Miki begins to develop feelings for. To make things more disturbing, everyone moves into one house. What will everyone think about Miki's family lifestyle?