3-days Marriage ~Hikaru Genji no Koibito~

Sumeragi Kouichi is heir to a lineage historically known to produce prestigious statesmen. In spite of this, he has remained outside the political spectrum, enjoying life as a normal high school student. Yet destiny has other plans for Kouichi, and his world is drastically challenged when its revealed he is to marry the daughter of a wealthy businessman: a girl who also happens to be his best friend’s younger sister. Can Kouichi find true love in something devoid of freewill; or is this marriage of convenience doomed to fail?

[From an old review by Nargrakhan]