Wedding dress shopping.. who to bring?

fiances mom and stepmom HATE eachother..stepmom was dads mistress at one point but they have since gotten married and had a baby. I am closer with his stepmom and I would like her to be a part of dress shopping as well as my mother, of course!! I also have a best friend who will be coming with... I just dont know if I should involve both or just one.. his mom and i have had a very tough time getting along because im like 99% sure shes bananas... still, its his mom and i have since been nice to her and things are repairing.. she has actually called me twice today but his stepmom is like a bff ... she invited me to help her register for her baby and I babysit him from time to time and then we go to coffee or lunch and such.. it's a tough choice but i know they wont go together... his mom almost vomits at the sound of his stepmoms name.. help!! GOSH ITS HARD BEING A BRIDE!! im about to elope in vegas in my HS prom dress!!!!!!!!!