Love Hina: Motoko no Sentaku, Koi ka Ken... Naku na

Out of nowhere, Motoko's sister appears and is ready to pass on the dojo. However, Motoko is still not quite as strong as her sister and when her sister finds out about this, she gives Motoko an ultimatum: Motoko must beat her sister or marry Keitaro! <br> <br> (Source: Wikipedia)