In an interview, FUSHIMI Tsukasa (the author of the light novels) was asked for his comments on the following:
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.
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:
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:
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.