Marmalade Boy (1994)

Miki's parents are getting divorced. Why? Because her parents bumped into another married couple, and found that each other's partners suited them better and plan to remarry. While this upsets Miki, the other couple's son, Yuu, doesn't seem to mind this at all. Now all six of them live in the same house and while Miki doesn't like this arrangement, she can't get Yuu out of her mind...