wait just to know in the resume its say that we can end with natsumi forever or a harem but in the start it say that we have only one wife and the sister is marrying a guy is that some sort of time travel thing ?
It's a glimpse of the future.
Legally you can only marry one person in Japan (where the game take place), and in that world, that law is still not changed after 11 years. Though there's so many reason to get married, and please don't assume anything until you really see it :)
You will either ended up with just Natsumi forever or with a harem.