There are lots of disturbing light novels out there. Magical Girl Raising Project, where the cast has a casualty rate that puts most zombie movies to shame; Ao Oni, which places the characters from a slasher movie into a timeloop where they die gruesome and horrible deaths again and again; and Grimgar, which is so bleak that the author's stopped killing characters so he can find even worse things to do to them.
But few can hold a candle to A Sister Is All You Need. This is a twisted tale about one of the most fucked up people you will ever meet.
Naturally, he's a writer.
But not just any writer. Itsuki is one of those light novel authors who churns out multiple volumes of multiple series per year, and every last one of them involve guys who want to bang their little sisters. Itsuki doesn't, so far as he knows, have a sister himself. His fetish is theoretical only. What cranks his motor is the idea of having a girl who is as emotionally needy as himself but willing to support him unconditionally.
So he writes every variation on the fetish he can imagine--and he has quite the imagination. So much so that his editor rejects most of his proposals as beyond the pale. The first volume, for instance, starts with an excerpt from one of his manuscripts that ends with the protagonist eating his sister's panties for breakfast, and that's not even the most outrageous example of his writing we're given.
Of course, the series isn't just about a guy writing wank fantasies. In fact, most of it is disturbing in completely different ways.
The plot, to the extent it has one, is a lackadaisical look into the life of light novel authors. Mostly this is indistinguishable from the life of nerdy college students--they drink more beer than is medically advisable, stay up late playing D&D, go on spur-of-the-moment roadtrips, and (very occasionally) hop on a computer to write.
There's Nayuta, a female author who would like nothing more than to screw Itsuki so hard his brains melt--something she makes sure to mention twenty times in every scene she shares with him. In the scenes she doesn't, she's usually harassing one of the other female cast members into getting naked and making out with her. Some of these scenes--and they're described in more detail than anything else in the series, and accompanied by illustrations that you wouldn't want to be caught looking at in public--go so far that they'd meet most people's definition of sex.
Her main target is Miyako, a friend of Itsuki's from college who has a pleasant but passive personality that makes her perfect for Nayuta to push around. Mostly, though, Miyako exists so the other characters can explain to her how the publishing industry works, and to be the target of a crush from Itsuki's other writer friend, Haruto.
Haruto is the guy who makes sure the good times are always rolling, providing both the beer and games for their gatherings. Despite that, though, he's a highly dedicated writer, turning in his manuscripts well ahead of their due-dates, and making effusive posts to social media. Sadly that hard work doesn't pay off. Though he's a successful author, he's not a superstar, and when his series gets adapted into an anime, it ends up sucking because the studio put all their effort into a more popular work. Honestly, he's far more sympathetic a character than the actual lead characters.
This cast is joined from time to time by other hangers on.
There's Setsuna "Jigglyass" Ena, the illustrator for one of Itsuki's series, who's renowned for, well, drawing jiggly butts. To hone his talents, though, he needs to find models, and he's not above pantsing a girl on the street to get a look at her butt. This is not only supposed to be funny, but when he becomes obsessed with finding her identity, it's presented as something we should sympathize with.
Then there's Ashley Ono, an unscrupulous tax consultant who helps Haruto and Itsuki deduct their porn expenditures as business expenses; Kaiko Mikuniyama, a manga artist who is obsessed with drawing characters in their underwear even when that's not what the story calls for; and Kenjiro Toki, the editor who has to rein all these weirdos in.
But most significantly there's Itsuki's step-brother, Chihiro. Except, as we learn at the end of the first volume, Chihiro is a girl. You see, Itsuki's dad is fully aware of what sort of books his son writes, so when he remarried, he asked Chihiro to pretend to be a boy around Itsuki lest Itsuki act upon his deviated preversions. But despite those precautions, it's clear that Chihiro is developing exactly the sort of feelings for her brother that he's always wanted from a sister.
Now granted, Itsuki and Chihiro are only step-siblings, and while Chihiro is still in high school, the age gap between them is only three or four years, but still, for a series that's trying to be a meta-parody, at times it's far too earnest. As with Hirasaka's previous series, Haganai, he simultaneously wants it to be a hilarious farce and a heart-tugging romance, with the result being a bit of a mess. Setsuna has straight up committed sexual assault. We're supposed to find it far enough over the top that we can forgive it, yet still take him seriously enough that we sympathize with his search for butt-girl. Nayuta is even more over the top, and her behavior often verges on sexual harassment, but Hirasaka still wants us to take her relationship with Itsuki seriously. Ditto with Chihiro's feelings towards Itsuki.
The result is almost paradoxical. If the characters are cartoonish enough that you can laugh at their bad actions, you can't sympathize with them; but if you take them seriously, you end up wanting half the cast dead. It's like putting pineapple on a pizza. Pineapple's good on its own, and so is pizza, but they absolutely do not belong together.