Before that, a harem meant a group of wives and concubines who are the property of a male ruler or other powerful figure*, so I hardly think the “harem girls looking at each other is NTR” crowd can be excused on the basis of “adhering to the ‘original’ definition”. I think they’re more so in a state of “having watched too many harem anime comedies”.being traditionally "a group of females constantly trying to one up another to win the male's full attention and officially become his only girlfriend"
* And before that, it was just a neutral term for the women’s quarters in a traditional muslim home.