Ok, I read Hot Gimmick and then immediately read this afterward. And I will admit I think Shinogu is the best guy of all of them and I felt badly for him at the end of the manga. However, Ryouki didn't turn out to be all that terrible (just awful at showing it), so I did like the ending of the manga too.
The novel did feel to me like it was written for middle-schoolers, but having not read it in the original language, I take that with a grain of salt anyway. I don't think the writing style makes it unreadable though.
If you're a Shinogu fan, then you'll certainly like this story. If you're a Ryouki fan, you might read it just to find out the details of how Shinogu came to be with the Naritas and why the Naritas needed so much money. (It wasn't to raise Shinogu.) And Azusa fans, at least in this ending
Azusa doesn't need to use Hatsumi in his revenge plans anymore.
The first chapter and the beginning of the second is a recap of major events in Hot Gimmick vol 1-11. The alternate timeline starts at what might've happened if Hatsumi hadn't run into Ryouki at the top of the stairs at the end.
I don't have a brother, and if I did, I doubt I could think of him that way either, but I do think that the situation was treated with delicacy and that the characters are still true to themselves in this story. In shoujo, don't we often suspend our disbelief with regards to romance? I don't see why we shouldn't do it here either. I think this ending was reasonably plausible, just as the original ending was.