This was a thing...
Though, before anything else, I suppose it's worth saying that the translation feels quite awkward in a lot of places... It's not the worst translation I've ever seen, but it's not great either.
Not that the quality matters that much, since for the most part, this is a pretty typical "guy gets turned into a girl and then gets turned into a slut" story. It even uses the cliche of "it happened because of an accident and they had to rebuild his, now her body into an improbably hot and sensitive bitch". I will, however, give them points for the fact that based on the events in the EXTRA section, it seems like the doctor did it on purpose, messing with both siblings to turn them into products to be auctioned off. So I suppose in some ways it's better than some other examples of this crap that I've seen before...
In any case, for the most part, the whole thing is sex scene after sex scene, and the True Ending is about as depraved and extreme as Dieselmine could afford to make it without crossing various lines... and even then, they implied that some of those lines were crossed off-screen, or could have been crossed, etc.
I suppose I should also give them points for how the little sister was turned into a futa, which contrasted with her older brother than became a pure girl. That was an amusing twist. With that, they reach near maximum levels of incest, especially if you include some of the things that are mentioned to have happened of-screen.
Would the fact that I can't decide if the True Ending is good or bad count for more points or less points? I can't decide... On the one hand, the two siblings end up as a mixture of sex-slaves and adopted children... on the other hand, they don't seem to be treated that badly, given that they get quite spoiled by the guy that buys them and his son, and both seem quite happy with the arrangement once they get used to it. The deadbeat dad that sold them dies too, but they never find out about that to my knowledge. Shame that the doctor didn't have something terrible happen to her, if it had that, I'd have put the True Ending under a good ending position.
Oh well, a pretty fun way to waste some time. The smut was decent enough for my personal tastes at least, so I can't complain too much, aside from wishing that the translation was better.