If you know the Sims' names you could use the resetSim 'cheat'.
I solved the issue by evicting the family off the lot and editing the lot.
Note to self: Spiral staircase cannot have a railing immediately beside the stairs. There need to be at least an open square by side of the exit/entrance.
To fix: Just broaden the platform by 1 square row on the second floor.
Sorry for being too fast to post the issue up when I could have edited it myself, I just wasn't very familiar. Anyway, whoever is playing this wonderful world could take note too.
A side issue: If there was another visitor sim in the house when I evicted the family, my hopeful guess is he will just be reseted into his own residence and not disappear from the world forever, am I right...? I would hate to lose that sim... haha