Difficult Bridesmaid

Hi Ladies, Im new to this forum stuff but thought i would give it a go for some help. I am getting married next year abroad and me and my partner wanted to get our family involved as we are really appreciative that they are paying to come abroad, although a lot of them are using it as there weeks holiday with their kids, so we have a big bridal party. I have 6 adult bridesmaids and a few flower girls. I am very aware that they are all different shapes and sizes so said that i wouldn't force one dress on them all and if they want they can all have different dresses, as long as it fits our vision, we have told them the colour, narrowed down the fabric and it has left 10 different choices for them all to chose from, which i sent to them.Just a bit of extra info) We are paying the majority for the dress and they will all end up just having to pay about £20 or £30 towards it. We are also paying for their hair to be done and again I have given them a selection of choices of 4 hairstyles for them to pick from, I've said wear whatever the shoes they want as long as they are silver. So all in all I think ive been quite accommodating. When I sent the 10 options, I received a few messages 90% were all great they loved them and they have found a few they want to try on, my one friend is the biggest and doesn't wear dresses at all and even she said they were nice and there were a few she will try. But i received one message for a member of my partners family telling me she hated everyone and didn't want to wear any of them as she said they wont be flattering on her and wont make her look good - she hasn't even tried these on yet - and 3 of the dresses are the same/similar to what she wore to my partners family member wedding already - now ive heard little snippets from my other bridesmaids that she is being difficult in other areas regarding my hen etc and making everyone wound up from her behaviour. She has known about the wedding for 2 years so she has known that she will have to wear a bridesmaid dress so surely if she had that big of hang up surely she could have done something about it. The thing is aswell, shes not a big girl shes a size 10/12 - I spent ages deciding on these dresses - there are about 7 different necklines, they are all long. Im now dreading the appointment in a couple of weeks and can just imagine her refusing to wear any dress!!! Im not sure what to do about it all, i suggested that she could wear some spanx type underwear and she said no - I told my partner and he said if she doesnt pick a dress tell her its one of these designs or nothing! I obviously dont want to be that harsh, but he said that he hasn't given his groomsmen a choice of what they are wearing so the fact she can pick from 10 dresses!!! What do i do now??