The author has been beating everyone on the head with the fact he'll "marry" all five of them. The hand grabbing at the bonfire festival was a pretty good bait and switch, where he leads us to believe Itsuki is going to be the main girl, only for all five of them to fulfill the "prophecy". Also, the chapter in the future, where those two women comment on the number of dresses seems to imply (at least to me) that all five sisters are going to play the bride during the day.
Hell, it would be incredibly easy to deceive everyone and escape social censure for them since all sisters look alike to the point few can tell one apart from the other. And finally, the author has done a pretty good job of giving all girls equal screen time.
That being said, it's not unheard of these kinds of stories to go wrong. Tsuki Tsuki comes to mind, and apparently Love Tyrant drops the ball in that regard as well, or at least that's what I've heard.