Sure but it all fits into the story.
They aren't having sex because they've been corrupted, they are having sex for other reasons that fit into what is happening.
That said, it does feel like the sister is becoming corrupted way too fast.
Still, it's got one of the more coherent stories for a sex VN so far. I'm interested to see where this goes. For a second I thought the scene with Minami was going to give you a choice to go good path or bad path, but it seems like it's still basically a VN on rails with not much choice for the player, yet.