A little bit of story info noted in spoilers on the main page isn't asking for the world especially when these devs want financial support.
I have seen countless arguments about this very topic we are discussing now simply because of that.
There you go, pushing the slippery ball down the slippery slope.
While this situation may be similar to the way most devs will warn players of NTR in their games, this absolutely should not be a standard, just a courtesy - a courtesy that devs do not actually owe us.
In fact, it's nice to be treated like a person with capacity for higher thinking: no bullshit, and if you're naive in making choices, you get punished.
This game made it clear from the start that MC will have to marry in order to eventually become king. And since the MC didn't lay the pipe on his sister or mother yet out of fear of narrative consequences, it's obvious LM is taking a more realistic approach to medieval fantasy... as ironic as that may sound.