Aki Sora

Aki Sora (あきそら, lit. Autumn Sky) is a Manga series written and illustrated by Masahiro Itosugi. It greatly resembles four chapters of the Hentai title Oneechan no Onegai which was written by the same author, but is not as explicit.

Aki Sora focuses on the relationship difficulties of one Aoi Sora, an effeminate young man who has ridiculous luck with women. However, most of these relationships are seriously twisted, if not completely taboo, not the least of which is his relationship with his older sister Aki and his younger twin sister Nami. His family also has secrets that are beginning to influence the story more in the most recent chapters.

An OAV has been released, adapting the first chapter of the manga.

As a result of recent censorship laws in Tokyo, the entire franchise was canceled in April 2011. This includes reprints -- the publishing company is claiming the new laws prohibit them from reprinting the previous volumes, putting the entire series out of print of early 2011.

Has a Spiritual Successor in Uwakoi, which has a similar premise, albeit with the incestuous bits filed off.

Tropes used in Aki Sora include:

Accidental Pornomancer
A Date with Rosie Palms
A Party - Also Known as an Orgy: In chapter 6 Runa invites Sora to pose as her boyfriend at a party... which turns out to be an orgy, and she has sex with every man there while he's done by the women. Ironically, they don't have sex with each other.
Author Appeal: The crossdressing and incest elements.
Barbie Doll Anatomy: The lack of genital details is the only thing keeping this from being a full-fledged hentai.
Body Paint: Chapter 11 -- Runa goes to the pool with a painted-on bikini.
Brother-Sister Incest
Can't Hold His Liquor: If chapter 6 is any indicator, Sora's alcohol tolerance is practically nil.
Cool Big Sis: Runa's Odd Friendship with Sora drastically expanded the horizons of his sexual-identity.
Crossdresser: Sora and Miharu. Sora finds himself engaging in this seemingly every other chapter, usually because of Nami's fetish and Runa's belief that he enjoys it. Miharu, however, really does seem to enjoy it.
Date Peepers: Nami to Sora and Kana in chapter 7.
Deconstruction: A rather brutal one of how most hentai/ecchi manga portray incest. Usually, it is portrayed as guilt free and sexy. This manga, while often making the second half true as long as the incest part doesn't bother you, really goes into detail how much the characters are wracked with guilt and fear of being discovered, playing it for as much drama and angst as possible. Also, the sex is also portrayed as disturbing at times, partially due to some of the characters being rather Cute and Psycho, and partially because the male protagonist is often a rather tragic case of The Chew Toy.
Double Standard Rape (Female on Male): Neither Sora's first times with Aki and Runa nor either encounter with Alice are exactly voluntary, and he's essentially conned into the orgy in chapter 6, but he doesn't put up much resistance for any of those. Nami unquestionably rapes him, and it's a very disturbing scene, but by chapter 15, when she later asks Sora if he hates her, he only admits to being scared of her... and both then proceed to have sex with no particular signs of trauma.
Dude Looks Like a Lady: Sora looks a lot like Nami, so much so that when they were young, even their mother had a hard time telling them apart... even when naked. In chapter 20 Runa is able to pass him off as female to the art class teacher just by having him wear her uniform.
Dysfunction Junction: This makes David Lynch's Blue Velvet seem like Pleasantville in comparison.
Hair Decorations: Toyed with. Nami normally wears her hair up to differentiate herself from Sora, but when she really wants to get Kana's attention, she lets her hair down to make the resemblance STRONGER. On the flipside, Sora sometimes finds himself sporting a set when the various girls force him to crossdress.
Fan Service
Half-Identical Twins: Sora and Nami.
Hot Springs Episode: Chapter 5.
Ho Yay: Sometimes Miharu and Sora look awkward.
Keep Circulating the Tapes: Volumes 1 and 3, starting in July 2011, due to Bill 156.
Lap Pillow: Between Nami and Sora in chapter 19. Aki wonders when they got that close, and Nami asks if it's so unusual for siblings to be close.
The Loins Sleep Tonight: Miharu reveals in chapter 12 that he can't get it up unless he's watching his girlfriend Alice get nailed by another guy. In chapter 18, as part of the game he, Alice, and Sora are playing, he has to "confess something embarrassing", so he yells "I AM IMPOTENT!" out the window.
Lovable Sex Maniac: Runa.
Love Dodecahedron
Naked First Impression: How Sora first meets Runa. Turns out she's like that all the time.
Name's the Same: Sora shared his name with a (female) Japanese pornstar.
Oh Crap: A variation; Runa was in the ecstatic throes of a climax as she loses her virginity to a total-stranger in Chapter 6... only to freeze in abject-horror when she remembers too-late that sex gets you pregnant as she suddenly felt the heat of his climax pouring into her womb.....
Plot Condom: Why else would Sora be able to have sex with women so many times without impregnating at least one of them?
Plot With Porn
Public Exposure: Runa in chapter 20, modeling nude for an art class.
Psycho Lesbian: Nami in chapter 8. After the date she set Sora and Kana up on goes far beyond her expectations, she confronts Sora that evening, shoves him down, stops just shy of cutting off his penis with some scissors ("Hey...can I have this?"), rapes him while fantasizing that she has a penis and is fucking Kana, and threatens afterward to cut his penis off for real if he ever tells anyone.
Rape as Drama: See the previous trope.
Really Gets Around: Sora, although he's so passive most of his sexual encounters were initiated by females.
Red String of Fate: Literal, as of chapter 13, but with Rule 34.
Scars Are Forever: Played with in chapter 17. One of the things that makes Sora start doubting that their mother is really who she says she is is that he vaguely remembers her having a burn on her arm from being scalded when Sora was a toddler, which seems to have vanished, only to be put at ease since he forgot which arm was scalded. Aki, however, knew the truth - their aunt scalded herself to simulate the scar... on the wrong (i.e., right) arm.
Screwed By The Youth Healthy Development Ordinance
Sibling Triangle: Triang Relations type 11, with Kana, Sora and Nami. Kana is Nami's best friend who fell for Sora ever sine their first encounter; Sora's affection towards Nami seems one-sided, while he likes Kana (but likes Aki more); and Nami has a Matchmaker Crush on Kana. It seems to have turned into a type 8, as Nami is warming up toward Sora after raping him...and proceeds to have consensual Twincest sex.
Single-Target Sexuality: Runa, despite all her...quirks, found out that Sora is the most necessary element.
Two-Person Pool Party: Aki and Sora in chapter 5.
Theme Twin Naming: Sora and Nami subvert this for being opposite... in everything.
Unusual Euphemism: From Miharu, ahem, "Catherine", "Dama Tower", etc. all the more disturbing with desu at the end of every sentence.
Unwanted Harem: Sora's, and it only seems to grow with each new character introduced.
Victim Falls For Rapist: Several of Sora's trysts are flavored with this.
Zettai Ryouiki: Standard for the girls' school uniform.

The following tropes themselves are spoilers; read at your own risk:

And the Adventure Continues...: Oh, the author has left himself a back-door open to come back when Bill 156 is over. See Sequel Hook below.
Broken Masquerade: In chapter 17, Sora begins doubting that their mother is really who she says she is (see Scars Are Forever above). Aki, on the other hand, has apparently known the truth for some time but hides it to protect Sora. When we see Sora in the final scene, it's clear that he's been made aware of what happened to his mother.
Not So Different: While using Sora and Aki's forbidden relationship as an excuse to take the latter away, it later turns out that their old man is their mother's biological brother.
Reality Ensues: Abruptly and hard in the last 7 pages.
Replacement Goldfish: Turns out the Aoi siblings' mother had a twin sister. Guess which one died in a car accident?
Sequel Hook: While on face value the resolution of this tale is one where Aki and Sora platonically break up, on closer inspection the author leaves us with all the (heterosexual) relationships reset-to-zero, unresolved and ambiguous; leaving Sora free to pursue anyone in his "harem." Though this is only a fleeting hope until if/when Bill 156 is dissolved.
Twincest: Forced during the Psycho Lesbian incident, but later done willingly by both twins.