Can't you turn on debug mode and forc an error on the stairs?
If not... then turn on build mode, use the hammer tool (if you have Pets) and delete the stairs. Then put back new ones. It might be better to use the multi tile ones (or whatever you call 'em) instead of the whole staircase ones. I vaguely recall that the tiled ones allows multiple sims to walk up and down it, because each individual tile is considered a separate object, I think
as opposed to the whole staircase being a single object (which as a result, is considered totally occupied or "in use" when a single sim goes up or down).
Ste