I hate you like a sister…

Since I can only use the word companionship so many times, I decided to look up synonyms to vary my word choices.
So as I’m reading the differerent shades of meaning for companionship – like fellowship or hospitality or partnership – I come across this entry for fraternity:

Main Entry: fraternity Part of Speech: noun Definition: brotherhood Synonyms: Greeks, association, camaraderie, circle, clan, club, companionship, company, comradeship, fellowship, frat*, guild, house, kinship, league, letter society, order, organization, set, society, sodality, union

Antonyms: sorority
Source: Roget’s New Millenniumâ„¢ Thesaurus, First Edition (v 1.2.1) Copyright © 2006 by Lexico Publishing Group, LLC. All rights reserved.
* = informal or slang

and it surprised the hell out of me. Sorority is the opposite of fraternity? I’m a firm believer in the existence of mean girls but I think that’s overstating the case for them, no? Surely sororities are also about camaraderie and companionship, fellowship and society.
I’m not sure if I think this is funny or disturbing. Or both.