I wonder why people keep putting real physical logic into a manga series. "no harem" cause "real life laws" last I recalled this is a fictional fucking manga and anything goes. I don't ever recall any Japanese girls looking like the quints that for fucking sure, especially with bodies like that and looks like that?
Usually harem stuff is just for the most part just shit like this, the quint girls liking fuu and fighting over him for his affection, it doesn't necessarily mean we'll get a "harem ending" as much as we want at best he could just make one official ending and then just other endings to please fans which probably wouldn't be outta the ballpark. More money for Negi and more fan-pleasing stuff for each faction of sister or harem.
That or a doujin artist/author does that job for us. This isn't a manga nor story with a harem ending unfortunately as much as we want it but the author could make another ending just like I said so it pleases the fans so everyone is happy even with the official ending