The jist of the story is that Nanoka falls in love(?) with her
brother. Now, don't go off thinking weird things. This isn't Sakura
Tsuushin we're talking about here. Nothing happens between the two.
In fact, nothing really interesting happens, period. Somehow, though,
it's not as boring as it should be from the little content it has.
The keyword here is "innocence."
I usually don't write details about what happens, but I don't think
that knowing the conclusion will affect the enjoyment of this manga.
Just about none of my readers will read it anyway.
So, in the end, both acknowledge their love(?) for each other. Nanoka
is truly innocent about it, but Koushirou is not a child any more.
Unlike Touma (Sakura Tsuushin), Koushirou does the right thing. He
decides to move out of his father's place and start living by himself.
It's about time for him to get on with his life anyway. As a final
request, Nanoka asks him for a kiss. Koushirou kisses her on the
forehead. It's a sweet little scene.
They both walk away from each other and that's the end. Actually,
Nanoka runs away with tears in her eyes, but the end result is the
same. If the story were to continue into the future, I bet they'll
reminisce about the whole thing with fond memories and nothing more.
***Update**
Apparently, there is a volume 4! And it looks pretty bad from what
I can gather. I'm going to pretend I never saw that and move along.
As far as I'm concerned, it ended with this volume and it was a good
ending.