The cover is misleading, since there is surprisingly little fan service.
Story: Big Sister goes missing, and her little brother decides to search for her when a lead finally turns up. That's more or less the overall story. With lots of melodramatic moments.
Art: For a manga that has such a weak plot, I assumed fan-service would be abundant. But the art being below average with pretty bad character design, maybe it's a good thing there isn't much fan-service. Highschool DxD at least has plenty of pretty ladies to carry their mediocre plot...
Characters: No comment. Since I literally can't even remember them.
Overall, this manga is half-assed. It's
not focussing on one thing, and that's not necessarily bad. But in a 4 volume story they can't simply waste time. Only reading volume 1 might not be enough to get an accurate impression of the story, but the "cliffhanger" was enough to show it's not going to be all about the missing sister. So its up to you if you want this story. Me, I will drop it and spend my money on something else.