http://myanimelist.net/anime/13367/Kono_Naka_ni_Hitori_Imouto_ga_Iru!
they haven't "revealed" who the imouto is yet but it's totally her
'basically he has to go to private academy for one year and take a crash course in 'running an international company' so that he can inherit his father's business (yes, apparently a one year course in managing a multi-billion dollar corporation exists) with his dad adding in the clause in his will that MC has to fall in love and get married before the year is up or he can't inherit
and of course one of the girls is secretly his sister who is concealing her identity because she wants to marry him'
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I can see a better time, when all our dreams come true
http://i.imgur.com/wITvQ.jpg