From NagareBoshi:

Yui has a deep love for her "older brother" (they're not actually siblings, but are related), and she is devastated when he gets married. While at the wedding, Yui meets a sobbing boy, Kaoru. It turns out that his cousin (that he was in love with) is the one Yui's brother is marrying.
When Yui moves into a boys dorm so she can seek out new love, she finds that the head of the dorm is none other than Kaoru!