What's this anime with a boy and girl meeting and becoming siblings? The story is about a girl and a boy meeting after their parents went on a vacation. When their parents came back they exchanged partners and the two became siblings. Please help. I wont be able to sleep well until I remember the anime. ;(
It's Marmalade Boy. Originally a manga, had an anime adaptation later.

From the Wikipedia page I linked above:


  Miki Koishikawa's ordinary life as a high school sophomore is turned upside down when her parents suddenly announce that they are getting divorced in order to swap partners with a couple they met in Hawaii. They seek her approval of the shocking change, and at a dinner where Miki meets the other couple as well as their son Yuu, who is about her age, she reluctantly agrees to the arrangement. Yuu, while at first a jerk who takes every opportunity to make fun of Miki, turns out to be fun and attractive, and Miki finds herself falling for him.