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Like the title says. I want a seriously shown brother/sister relationship, like in Flower of Evil or maybe Angel Sanctuary. It can be a feeling only mistaken for love. However, nothing too sweet.
What I do not want: yaoi, yuri, age difference larger than four or five years. The couple should be aware that they are blood related. They can be half-sister/half-brother, love triangles are okay.
And, is Boku wa Imouto ni Koi wo Suru worth reading?

Try Aki-Sora.

Yes; it fits your requirements and develops adequately. In the recommendation section for that manga you can find most other serious incest manga, like Tsumi ni Nureta Futari. Also check Lament of the Lamb. Finally, one of the arcs of the anime Yosuga no Sora has incest; there is also a manga adapting that arc, but I have not read it so I cannot comment on its quality. The anime was fairly good.

Koi Kaze. That is all.

If you are wondering whether to go anime or manga for that, I heard that the anime compares favorably with the manga, and I heard people say it's better, even. However, I haven't read the manga, so I can't confirm that.

Koi Kaze...Koi Kaze and Koi Kaze.

By far the best of it's genre. It is so well done that I can't express it clearly

Several people recommends you Koi Kaze.

lol
*ad music*
Koi Kaze: Recommended by over several people!

Edit: seriously now, Koi Kaze is by far the best of its kind, but notice the OP's age difference requirement.

Koi Kaze:
Born twelve years apart, siblings Koshiro and Nanoka were separated when their parents divorced and moved to different parts of Japan; as a result, they both grew up hardly aware of the other's existence.

OP:

Even though the OP asked for manga. It's that good!

Koi Kaze IS a manga, but it doesn't fit her requirements.
http://myanimelist.net/manga/1331/Koi_Kaze

Oh and I don't see Angel Sanctuary on your list, but considering you mentioned it I assume you have read it.

You could also try Ayashi no Ceres- the incest however is... Weird, and not really about the incest.

I know, but the people who recommend it have only seen the anime.

I didn't see that part about the age gaps, did I? Naturally, I wouldn't have recommended it if I did.

Since someone recommended Koi Kaze, you can also recommend Boku wa Imouto ni Koi Wo Suru, People seem to prefer it over Koi Kaze because they say it has a better story line, characters and art work.

Seconding Aki Sora, It has quite a lot of sex scenes but it also sheds light on the emotional struggles of the characters. Just be told, it was canceled because of
Ishihara's Law.

Is that true? I didn't know that :(

Kiss x Sis? lmao

She wants something dark, not something that will cause her to burst her gut open laughing.

There is an arc in Black Lagoon about very..... let's just say 'weird' twins. My memory is a bit foggy but I think the arc started at the second season (Black Lagoon: The Second Barrage). There is also a manga of Black Lagoon but I'm not sure how closely the anime follows it. Anyway, in the end it's just an arc and the first season is more or less just for mindless fun (but it does it very well).
Just wanted to recommend something 'different', I don't think there are that many manga that fit your request :|

Thanks! :) I'll surely try Lament of the Lamb, Aki Sora and Boku wa Imouto... .

I've seen Black Lagoon: The Second Barrage (by the way, it is truly awesome).

@Amebrleh: I have Angel Sanctuary on my list, MAL uses its Japanese title: Tenshi Kin-something.