Why i think Harem ending is a bad choice:
given this is a Harem manga, there cannot be any real love rival for Yuu, since otherwise it would be a different genre. Given this, the only real conflict a Harem manga can have is "who is going to win?" If you go for true harem, you nulify the only real driving force of the genre. Of course you can make a really cool story about the conflicts and issues of a polyamory relationship with 3 sisters. Pretty sure there would be a lot of fucked up shit you could explore. Not with this fluffy feel good manga, tho.
Therefore, i'm guessing if it really ends up being true harem, this will get really fucking boring.