I'm actually kind of surprised there isn't a review for this manga yet. I'm just going to make this nice and simple, seeing as the chapters come out pretty slowly and I don't remember too much about the beginning. Story: 9 I remember I felt as though this manga had a pretty slow start. Not much romance was happening at all. It was more like, a long stream of steady character development, but it wasn't enough to hold all of my interest. Eventually, the romance started to sprout. And I love the way this story goes about it. If I could find a way todescribe the approach to the romance of this series it'd be... mature... and delicate. It's most certainly slow. And as I always say; The slower the romance, the better! (Slow romance tends to have a lot more development and meaning to it.)
The story is about a young, I believe 15-16 year old boy and a woman in her mid-twenties, who end up living together because the woman (Narumi) is working for his mother, who is a manga artist.
Eventually throughout the story the two begin to learn more about one another, and even start to change the way they live because of it.
For example, Narumi actually starts drinking less and less throughout the series because Jun told her he doesn't want her to drink anymore.
After a while, Narumi starts to noticeably gain romantic feelings for Jun, but she's actually partially conflicted to feeling this way, due to the difference in their ages. So she keeps it to herself, only to at times show her true feelings ever so slightly in situations that call for it.
The same goes for Jun, though he's a tad more clueless about his feelings than Narumi is.

Errr, I better stop before I end up summarizing everything that has happened so far.
The story is very good and approached in a well done manner. It's one of the slower romances, which means it's one of the better kinds of romances.

Art: 7
The art isn't the greatest thing in the world, but it isn't bad either. I'd say it's straight out "good" and nothing more or less than that.

Characters: 8
Jun, the main protagonist, isn't really why I'm giving this section an 8. I'd say it mainly goes out to Narumi, and a couple of Jun's friends, because they're all simply charming. Narumi is very well developed and it is interesting to watch her struggle with her inner romantic conflicts when it comes to Jun.
Plus she wears glasses. That's a big plus.

Enjoyment: 9 and Overall: 9
Kono Onee-san wa Fiction desu!? was a bit slow at first, and I believe my first rating for it was around 7 or so. After a while, it's grown to a 9, and a strong 9 at that. I look forward to every new chapter, and pray that it isn't ending soon, seeing as it has tons of potential.