Nothing is worse for the adaptation of a still-publishing novel than a filler ending, especially if the anime turned out to be rather popular, as is the case with Ore no Imouto. The problem with filler endings of course, is that it's extremely difficult to make a sequel that is still faithful to the original material.
I imagine the producers at Aniplex were quite peeved and banging their heads against the wall about 6 episodes in, when they realized that their anime could be milked for at least another season. But what can they do now? There isn't enough episodes left to change the filler ending. Then one sensible producer, whom I will dub Jim, pointed out that there is this thing called the interwebz, and they could just post new episodes up there, if only for the sake of continuity. And all the studio were happy and had party that night, and Jim got a big promotion.
If you watched the original 12 episodes and liked it, then you'll probably like this, because it's more or less the exact same thing, only a slight different (and longer) ending. This arc DOES feature Kuroneko over Kirino, so if you're a fan of her then go wild. And of course to fans of the novel, this IS a faithful adaptation, so if you didn't like the filler ending then you'd probably like this one. Everything else, art, music, characters, still maintain the high level of quality as the tv series, so it is still the Ore no Imouto that we know and love.
Nothing else to say really, except with this new ending, hopes for a second season are high. *fingers crossed* =3