All I can honestly say is do what you and your FI want to do, it is your wedding. Stick to your guns and eventually people will understand and if they don't then there really isn't anything you can do about that. Please don't listen to your step mom, but if you are really worried about it you should bring it up with her and/or your dad. What she said really doesn't make sense, how could she not even think about you FI family?
From the beginning we knew we wanted a DW, we are from different hometowns, and neither of us are really rooted there anyways, so we figured it wouldn't even mean anything to us if we got married at home. We were both prepared for peoples complaints and excuses not to come, and eventually understood it was typically all part of having a DW. Now reflecting back, I couldn't imagine getting married any other way! Everything was what we wanted and it created such a relaxed, exciting and comfortable feeling for us on our wedding day
Maybe try talking to your families and explain to them why you want a DW. Good luck and enjoy your planning, things will eventually calm down