Oboro is correct. What she's suggested here isn't a harem. A harem is entirely focused on serving the needs of one person. She's suggested a full-on polyamorous relationship. Which tend to only work if everyone is involved with everyone else in it, and becomes mature enough to discuss and suss out jealousy the instant it arises, until it ceases to be an issue.
I can't hate Hibari for her selfishness because it's not just Hibari - it's everyone else at the inn aside from Oboro who feels the same way. They will literally sacrifice their friends and family so long as they can strut around with the MC.
After all, what happens if he picks just one of them? They'll have to move out of the inn, or the others will, because no one will want to sit around and get fucking cuckqueaned day by day, watching the love of their life be off-limits and wrapped around someone else only a few feet away. They might remain friends, sure, but it'll be a very distant friendship from that point on, sacrificing all the good times they've had the way they're all living now.
Harem or poly is the only ending that will make everyone happy. Oboro's got the right of it.