That's the point. If we allow reverse harems and all other types of harems, as long as there is a main person being liked by a bunch of other people, then the OP's sister should also agree that if a bunch of girls like a single gay guy, then it is considered a harem. Since she believes this actually does happen in real life, then harems can exist in real life.
@AURON570:It just occurred to me that OP's sister said that harems would not work in real life because even if a bunch of girls liked a guy, the guy would either be gay or turn gay. But wouldn't this still be considered a harem even if the guy is gay? or even bisexual or a hermaphrodite? Or does the guy HAVE to be straight and/or fertile?
What about reverse Harems?