In the case of the ML, he has been stripped down to his most basic form: has a name, barely a face, and a modifier to the base personality of "somewhat kind". Oftentimes this modifier is simply "otaku" to prevent alienating an otaku audience.
We then have the FML, who has instead been assembled into the widest-appealing fetishistic ideal. She is a sister but a girlfriend, an otaku but a gyaru, provocative but chaste. Much like ML, she's no longer an actual character, but through opposite means.
To me, this is a symptom of art's plight under modern capitalism. The audience no longer looks at art for emotional fulfillment, but rather as masturbation. It is simple consumption to them, much like McDonald's or a 2000 calorie Crumbl cookie. And what does this mean for the author? The author is incentivized not to create mindful art, not to pull from lived human experience, but to look only at other media.
Audiences don't want real and nourishing, they want quick and easy... or rather, they've been conditioned to want that. As a result, many a romance manga are now trapped in a simulacrum, slop imitating slop imitating slop imitating slop ad infinitum.
Most people who fetishize sisters don't actually have sisters. The magic 8 ball I shake for media analysis tells me that the point of the little sister type heroine is to give the audience a character who is affectionate from the start, without the need for additional investment. And also, the taboo element.
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Deleted member 206441
I hate to say it but when readers get to a certain age they gotta stop reading shounen and shoujo and move on to seinen and josei.