Well... this one was an interesting one in some ways. Honestly, I think the overall idea could have gone better overall, but as usual Atelier Sakura didn't bother fully working out the story to the max.
The fact that both sisters are stupidly sensitive and have a predisposition towards hard poundings and wild sex makes it a bit meh... but I will give it points for the fact that the driving force of all this isn't the guy doing the fuckery, but the older sister. That was honestly an interesting angle that really should have been explored further. Unfortunately, as usual for AS, they don't properly explore why Yumi and Mia care for the MC, or why the former remains obsessed with him even after she has mind blowing sex with a rich guy that's willing to spend obscene amounts of money on her.
The fact that Fuuma isn't particularly terrible as a guy, just has twisted values, makes it even more confusing. It's pretty obvious that he didn't force Yumi into things, and it's also obvious that if Mia actually seriously tried to resist his advances, he'd have stopped. She never does, of course, because like most girls in these things, she has nympho tendencies, is super sensitive, and was secretly always waiting for a guy to fuck her wildly and creampie her without a care in the world.
It still doesn't explain why Yumi still cares about the MC enough to cause that whole mess.
Of course, the most pathetic thing here is McCuck-kun, who is in many ways extra pathetic. As others have noted, his dick isn't smaller than the other guy, maybe a bit thinner. More importantly, in all the scenes, the girls never really say that the other guy's dick is OMG so much bigger or whatever. No, it's all about HOW they're fucked, not about the dick. So the real problem is McCuck-kun's skill at sex that isn't compatible with the girls, his unwillingness to really lay into them and all that. Now, there's several things that make him pathetic here. First, he doesn't learn shit from what happened with Yumi. Second, when Yumi pops up again, he clearly still has feelings for her (he doesn't really try to resist her advances the first two times and it's only when Mia's on the line that he bothers if you make that choice for him). Third, he doesn't even stop to think that someone with values as twisted as Yumi's might end up dragging her sister into something screwy and doesn't properly address the problem until very late in the game.
Honestly, odds are good that if he strongly rejected Yumi from the get-go, Yumi might have actually left Mia alone... but of course, he's way too pathetic for that.
Makes me seriously doubt that he'd be able to keep up the relationship with Mia after she was awakened to being a sex-addict. Sure, he seems to have finally learned something out of the whole mess, but I dunno.
The most disappointing thing about the whole thing though, was the threesome in that one ending. It was lame and too short. They really should have put more effort into that scene. More than one creampie for each girl. Better interactions between the sisters. Something... What we got was so lame...
Ah well, not the worst way to spend some time, and for me, in some ways, this was a better AS release than some others I've seen.