That is so sad! Your step-mother should have told you that she was going to get rid of it before she just threw it away. That way you could have had a chance to hold on to it, even if you had to put it in your closet or somewhere.
Your step-mom is a bitch for not understanding why you would hold such fond memories and feelings with that blanket, and I am sure your relationship with her will never quite be the same. So sorry, OP. FYL!