Need advice with step-mom?

the other night my stepmom brought over a wedding dress she wants me to wear...its not my style (it has long puffy sleeves and a 12 ft train) when i tried the dress and veil on she pulled a layer of the veil over my face (like some brides do and their dads uncover their face at the alter) but i can't stand things close to my face and took it off. well she slapped me HARD! in front of my finacees mom and sister (my MOH). i'm scared to tell her i bought my dream dress and i'm wearing it at MY wedding. i'm scared of getting slapped again and her making a fuss at the wedding. i thought about not telling her and just letting her find out wheni walk down the aisle but that seems way immature. my future in-laws said next time they see her slap me they will step in and say something to her. since i've known this women she always slaps me when she gets mad or doesn't get her way. advice please!!