Oh, I understand that, and I don't have a Problem with the incest in games. What bothers me is the cheating. It's not the scene itself — I’m fine with that; it’s well done.
But let’s consider the situation. As the main character (MC), we put in the effort, completed tasks, and built trust to win her over, right?
Then, without warning, she gets involved with her sister, and we, as the MC, know it's happening, but she doesn't and it just feel like a breach of trust, it is one. So, why should I care about Linda at that point?
If her sister had said something like, "Hey, don’t you want him to see this?" I wouldn’t mind, because that would mean we’re included and aware of what’s happening. But that didn’t happen, and that’s why it feels wrong.
How could I be okay with that? She tells the MC she cares about him, she loves him but then does something behind his back without saying anything, and then acts like nothing happened. That’s not just wrong — it feels deceptive.