Sims should definitely react differently when their own child gets "too" house warm, rather then when a stranger does it. It's no big deal to me though, as my students never visit their family during a term. But this also counts for relatives, like adult siblings or old parents visiting your home. In RL I might think to myself that a sister could kind of ask before roaming my fridge, but if she did starts cooking without asking me beforehand, it would any way harm our relation way less than if a stranger did the same.
Maybe the easy solution would be to have the option to simply ask permission to cook, sleep and all that other familiar stuff. Then, the house owner would more likely accept if the one asking is a close relative or a close friend. Both relation, mood and traits could affect whether permission was given or not. It's a bit more technical than normal family life would be, but this is a game after all. And - a chance of a challenge would just add to the fun.
