The game page is pretty sparse with info and I think I will be here too.
This was a pretty uncomfortable read, for all the intended reasons. You suspect the entire time that something is wrong, and you are trying to figure out what exact kind of wrong, until it the other shoe finally drops. The whole time I kept thinking "they're not actually gonna go there, right...?"
That said, I think I liked the premise more than the execution. I think the story is too short to let the dreadful build up simmer enough. While the brisk pace is disorienting and fitting, I think it looses a lot of its bite cause it's just too quickly over. It did build up the feeling of unease well, but I think it should have gone much further. I also don't think we had enough time of the girls hanging out and bonding for me to believe that Lucy would lower her guard down, or for Mary to grow attached to her to have second thoughts. The brother was a sparse character but have a very strong presence. For a story about manipulation, it didn't really leverage Mary's explore with either character that much, and that's a shame.
The interplay of the two endings was done well. I like the idea of going with the plan remaining implicit, although I think I would have gone with more disturbing implicit vagueness here, and having the other ending be the one that reveals the whole grim picture.
Lastly, I wish the game touched upon the Greek myths some more. There is an entire very rich pantheon of stories regarding fucked up shit that this story touches on to draw parallels to.
All in all, an idea that could have been really strong if it committed to slow cooking the horrifying implications.