I played TS2 a lot, but don't recall the sleepover feature. I did not have the apartment pack, though.
Anyway, when I want a sim living in San Myshuno to spend a weekend with her parents in Oasis Springs, I just use the household manager and let her join the old household temporarily. Sunday night I will move her back into her new city household. If she lives alone I will place a random placeholder sim in her city flat, otherwise I guess the game would think she is selling it. I don't think this routine is problematic at all, but of course the game will not see her as a visiting relative, but a full worthy household member.
Features that you request might be good for a future gamepack?
