In the episode about the keg party all the kids decide to have the party at Jackie and Kelso's secret "make-out spot" which is an empty pool at an house that is for sale. During the party Jackie then begins to freak out telling everyone her mom is on her way to show a couple the house because apparently she is a realitor. When Jackie's mom shows up with the people she is clearly not Brooke Shields but in later seasons Jackie's mom is played by Brooke Shields and there is never any evidence that Jackie had a step-mom.