In an interview, FUSHIMI Tsukasa (the author of the light novels) was asked for his comments on the following:
12巻は全体的に美少女ゲーム（ギャルゲー）を意識した構成になっていて、原点に帰った印象を受けます。
I felt that volume 12 [the last volume] was written in a manner informed by galges. It felt like a return to the way it originally was.
Fushimi responded:
そうですね。狙って１巻っぽいシーンを書いています。すべてに決着をつけて禁断の道に突っ込むという痛快な物語は、電撃文庫で、直接描くのは、とても難しいですし、桐乃を幸せにするエンドにしようと決めた以上、彼女の今後には、可能な限り良いレールを敷いてやらなくてはいけないし……非常に悩みました。結果は、読んでいただいたとおりなのですが、再読する方向けに、蛇足ながらコメントさせていただくと、
・「完全なる桐乃エンド」
・「最初の人生相談と同じように、兄妹は、二人だけの秘密を抱えて終わる」
１２巻は、そんなお話です。
I guess so. I wrote so that it would have scenes that felt like those in volume 1. With Dengeki Bunko [Fushimi's publisher], it was very difficult to write a thrilling story in which they settle everything and head down a forbidden path. I was sure I would give Kirino a happy ending, so I did my best to set up as good a situation for her as possible, for the sake of her future. I found it very difficult to do so. It ended up the way you read it [in volume 12], but if I were to add a few unnecessary comments for those readers who are reading it for a second time, I would say:
"A Completely Kirino Ending"
"The story ends with the two siblings sharing a secret, the same way their first life-advice-talk [jinsei soudan] began."
That's basically what volume 12 was about.
In another interview, Fushimi was asked:
すでに最終巻を読まれた読者には既知の結末になるんですが、エンディングに向けての展開はどういう風に決めていったんですか？
The ending [of the anime] will probably come as a bit of deja vu to those who have already read the novel. How did you decide how the story would reach its end?
Excerpted from Fushimi's reply:
[...]桐乃の想いが完全に報われる結末が[...]ひとつもなかったんです。普段表には出さないし、いろいろな事情があって、分かりやすい形で書いてあげることもできないけれど、彼女がとても主人公のことを想っているのを、僕は知っていたので、何とかしてやりたかった。[...]
In the main story [as contrasted with the PSP game], I couldn't write an ending where Kirino's affection is truly and entirely rewarded. Various circumstances prevented me from plainly writing that sort of ending. Nonetheless, I realized that she cared deeply for [Kyousuke], and so I felt I had to do something for her.
Fushimi's comments strongly suggest that he wanted to write a different ending - presumably a more "taboo" one in which Kirino and Kyousuke remain in a relationship - but was prevented from doing so by his publishers. Hence the "until we graduate" provision.
