Dear Stepmom:  When you hear of her vitriol reported by others, reply to them, "We're sorry to hear this. She's chosen this path and it breaks our hearts.  We miss her and her family."  Keep repeating.  You can't change her or help her till she's ready to realize her pathos.  But you can counteract the mis-characterization of you and your husband.  It may help the grandson, I'm sure he's wondering whom to believe.  So sad, I'm sorry.