WARNING: POSSIBLY DEPRESSING THOUGHTS/SPOILERS!
Honestly dude, I'm getting the opposite vibe more than anything.
It feels like you can say "No" to everyone else and just avoid any adult situations, and end up keeping your virginity, this seems to be what "your sister" expects/wants/asks/demands of you. (I'm not gonna say my suspicions, we ALL know what this game is drawing inspiration from, we ALL know things are gonna get "weird" to put it overly mildly, to say the least.)
In opposition to that, it seems like we can also just go full harem, except it's not a harem, it's just everyone using everyone else for their own purposes, even it's it's really only for pleasure. Honestly, it's a decidedly "different" situation than any other "Harem MC" I've ever seen, to be sure.
Honestly, I can't really see any point/purpose/reason to sticking with JUST one girl, nor have I seen much in the way of that actually being something the game intends as an option. If anything, it seems to go out of it's way to suggest either an "all or none" approach to the ladies. I really haven't seen anything to make me think that you can be committed to JUST one of the ladies (aside from, as I said your "Sister" by virtue of avoiding ALL the other ladies).
That having been said, you CAN turn down everyone else and only accept what a specific girl offers, but as the game is in this build, there's not much incentive to do so. With the only possible exception being the nanny, the MC clearly is NOT in love with ANY of these ladies, and only the nanny and MAYBE the "Princess" have/develop any real feelings for him, and how "real" those feelings are is entirely debatable due to the fact that there is CLEARLY something very "wierd" (that spelling IS intentional, check the source material!) happening to the residents of this "island" that is clearly influencing their emotions, libidos, and thought processes.
Ultimately, this is most definitely clearly NOT a love story.
I really don't think any of us should expect anyone in this game to get any real kind of "Happily Ever After" in the end.
Sorry for all this, but after 10 days, I just had to speak my mind.
FAPPY HORNYDAYS YA FILTHY ANIMALS! XD