6.5/10. The "moral" of the VN isn't reflected in the story at all, there are numerous unrelated story issues, and I personally just didn't care for the setting at all. All that aside, I have to respect the fact that this is free, and the dialogue can be funny at times.
The sibling experiment is--well, first of all, it's free. For anyone looking for OELVNs of slightly higher quality than Winged Cloud/Dharker tier, that's really all you need.
So I'll be honest, I didn't even realize it was a free-to-play VN while I was reading. There were a couple of things that I'll mention in a moment, but nothing was egregious enough to make me doubt that I'd paid for the experience.
In terms of some kind of "objective" metric, or a quality difference rather than artistic one, the only element that seemed poor to me was/were the CGs. They all looked pretty poor. Outside of that, however, anything I disagreed with felt like a stylistic choice.
...and there's quite a bit. The main thing is a "fellow kids" vibe I get from the VN. Most of the music some kind of hip-hop infused weirdness, a lot of the dialog is too cool for school, a character vlogs complaining about her brother, I personally find the clothing of most female characters (including (especially) the clothes of the parents) to be incredibly trashy. It's kind of interesting to think about, because it ties back in to something I was thinking before I got very far into the VN...
This isn't just an "original english language" visual novel, but it's also one that's [i]not[/i] set in Japan. One guy with a car listens to heavy metal and plays DnD. A clique of girls goes to the shopping mall. For a genre with a fandom obsessed with Japan, this VN does not fit into that category.
And while in and of itself I think that's a good thing, it can also show "us" (VN readers) our own biases towards Japanese VNs (JVN for now). That's not to say I blindly accept everything that happens in JVNs: I roll my eyes when the MC works himself up to charge into a women's bathroom so he can "mistakenly" see a girl in underwear, when LITERALLY EVERYONE in the story isn't repulsed by the MC dating a loli/his sister, when tsunderes tsun, when a character says "it can't be helped." But I'm more willing to overlook these things than some of these superficial elements in the Sibling Experiment.
That's not to say that everything here is the result of these subjective differences; the story has a couple really big problems. If the store page didn't make it obvious, you play two separate routes as two separate characters (who gradually interact more with one another).
One danger with this is that internal dialogue doesn't fit: one of the two characters has a very laidback personality (she calls everybody "dude"), and her internal dialogue reflects that quite well. The other character is very prim and proper, but her internal dialogue doesn't reflect it too well; it feels like the writer wasn't quite sure how he wanted to represent it.
Another danger is that characters portrayed in both routes act differently, and this does indeed happen. At the beginning, a character for each protagonist acts in a certain way that the protagonists respond very negatively to. They're then whisked away from this character for the majority of their remaining story...whisked away to the OTHER protagonist's related character, who does NOT act the way they did before (person 1 and A start together, then 1 moves to B. Person 2 and B start together, then 2 moves to A. Neither A nor B act the same way in the second half that they did before).
Along similar lines to the above, the "moral" of the story is something like "you don't know what you've got until it's gone," but the story doesn't actually show this: it doesn't show the characters missing what they had, or disliking their new lives. This is said, but never shown.
Yet another issue is related to the big mystery of the VN: the source of the two protagonists switching places. The story manages to present a couple situations that make this (how and why did we switch places?) seem like an important question, but it never actually gets answered. During one of the several endings available to you (kudos for that, by the way), the protagonists meet up and talk about it, but neither of their two (competing) theories are validated, and other related information is never addressed either.
In addition to these issues there's some disjointed-ness that's likely the result of trying to deal with player choice. Characters reference events that only happen if you make certain choices, a couple times characters seem to lose agency to force a plot point to happen, and during one specific instance an event that ought to go differently for the two different routes (the protagonist you're following asks a character close to them (different for the two protagonists) to do something) instead plays out the same (only one of the two characters "close to the protagonist" shows up).
