And "Always Welcome" reward trait is how one does that. It's inexpensive for a reason.
If you have the reward points, you can give your Sims the "forever welcome" reward trait, which will let them cook, shower, etc. at anyone's house. (If you don't, you can use cheats to give yourself the aspiration points to buy it.)
I agree that family members should be allowed to sleep or cook, even if they've moved into their own home. I also find the list of things that are "inappropriate" strange.
Random Sims visiting my home: Eat Lobster Thermidor out of my fridge without asking if it's okay or not (what if that's for a party tonight?) and then take a mud bath in my tub.
My Sim, after spending the night at their fiancee's home: Decides to make fiancee breakfast while they sleep in and is thrown out for being "inappropriate."
I think not just family members. But everyone in general. I will give you an example I have this one character she took on a task of becoming successful with zero money. To survive I have to couch surf her at neighbours because all she has is property. However this issue happens with her too and she's not a family member which I think is ridiculous. I think there should be a button something that will say "Grant permission" "Ask to crash" "Ask to use household" SIM can ask permission after developing a regular average relationship with the person or already having a relationship developed like with family members. And so then the household can give them permission stay or hang out for as long as they want. They could add options of how many days or for how long etc. I honestly thought that the button that says give residence key was doing that. I thought that if I give somebody my residence keys this already means that they can come in and use the fridge and do whatever they want because that to me felt like established trust.
