The Cuckoo is a story written by shukyou. It ran in Issue 75 at http://www.shousetsubangbang.com/mirror/the-cuckoo/ .
A missing child is returned to his family and tasked with the job of finding out why he disappeared in the first place.
You would not believe how much of this is based on a true story.
The imposter in question is Frédéric Bourdin, a French man who impersonated a missing child in Texas in 1997. There's been lots of mediocre movies made about this incident, apparently, but the first I'd heard of it was an episode of the podcast The Dollop titled 'The Chameleon'. What really kicked my interest into gear was Dave's theory that the family was somehow responsible for the boy's disappearance, and that hiding the murder was the reason they'd gone along with the obvious fakery. From that moment, it was only a matter of time before it got the SSBB treatment.
From there, of course, a lot of things changed: Romania instead of France, South Carolina instead of Texas, 1992 instead of 1997, a helpful older brother instead of a murderous older brother. And of course, even though it's October, we get a happy ending! We just have to wade through some creepy-ass sex to get there first.
There's no butler, sure, but it is a murder mystery, and I figured that's got to count for something.