Why does my stepmother resent me so much ?

Don't allow her to ruin the relationship you have with your father and the relationship your son has with his grandfather. Perhaps this like you said, she does want your father all to herself. She may resent the bond you two share. I know being nice to her continuously is easier said than done but I still think that is what you should do. Soon enough she will realise her actions have no effect on you and may stop. All the best with the move and everything