Modular stairs in Sims 2 have been buggy since they released Mansion and Garden and they never addressed it. I haven't had a stair bug (yet) since switching to Legacy, so I don't know if any of the things that used to work still will but here's what I've got:
Turn on MoveObjects on.
Find the right spot on the stairs to sledgehammer; trying different heights and angles.
Using the stair tool, try holding control while you click on the stairs, trying different spots on the stairs if it doesn't work the first time.
Move the lot to a new location.