People in blended families please help,my husband tom's exwife showed up at our x-mas party?

tom and i just got back from our yearly x-mas party at my inlaws house.when we were there tom's exwife showed up with thier son to bring gifts(even the dog got a gift but not me what a surpise) and made herself right at home for more than 2 hours.i know what everyone is thinking that i'm a evil stepmom.but what you don't know is we had a party with joe's son the night befor with my inlaws to exchange gifts with him.she knew we were having our party and can just to make me upset.we do not get along at all she is very bitter and jealous that i married her tom and makes my life hell anyway she can.my inlaws did not invite her but she uses thier son like bait to get into these functions.i'm also mad at tom as this did not bother him at all and he has no plans on saying anything to his ex about this.i told him how this made me feel and he just said what do u want me to do i can't stop her from showing up.i'm just afraid that she got away with it this time and will do it all the time now.