I think it is with InSim, I click on a sim, with another sim selected, and there's an interaction named selectable, I click on it and then there is make sim selectable and make sim unselectable.
I make my sims selectable using both BoolProp and the inSim ways, and neither way will let visiting sims interact with the fridge/mirror/dresser/phone etc.
I always could before I installed OFB. I've even considered uninstalling OFB due to this and other issues, except I'd lose a whole load of gameplay and a couple of families if I did, (and that Bender servo that I downloaded today from MTS2 - he's awesome!)
I love the inTeen, but I'd really like to be able to have my sims use mirrors etc too. *sigh* I'll give that mind control mirror a go though...
Edit: The mind control mirror works great! Now I don't have to choose ^_^
Atom XX