You might want to really involve a lawyer or at least consult one. Maybe not for your first inquiry but sooner than normal. You are right in that something is not normal here.

Normally an illegal tennant would be removed prior to the property going up for sale. Otherwise you may have problems or incur real costs getting her out. She may not even be an illegal tennant.

At this point in time most realtors are little more than used car salesmen in my eyes ethics wise.