I adore things like this, little tidbits of cultural behavior that run directly counter to patterns ingrained so deeply in me they predate my memories of being taught them. It’s rude to say “please” and “excuse me” to family members? Like, they’ll be offended by it? That’s awesome! I had never thought about viewing familial intimacy through that kind of lens, and it’s kind of like trying to put something in a box by looking in a mirror. I can understand the rules that govern what I’m seeing, and they totally make sense — but man, if I stop thinking actively about it every second, my brain tries to revert to what it knows is “right.”
I usually talk about cultural relativism in the context of extreme things like human sacrifice. This is a nice, small-scale example that doesn’t jump up and down on top of the ethics and squick buttons. It may look like Opposite World from your perspective, but that doesn’t mean it’s wrong.