Grooms` Sister- Obligated To Be A Bridesmaid?

I have a problem. I am getting married in August and my fiance and I have been paying for the wedding ourselves. Around Christmas, my fiance`s mother told us that she would help out with the wedding, and since she was going to, she thought it would be appropriate if her two daughters were inluded. I thought it was fair and allowed them to be junior bridesmaids, along with my two neices who are also junior bridesmaids and four close friends who are bridesmaids. Now that I haven`t seen any money, and the sisters have never answered my bridesmaid emails I sent to everyone, and never even seemed excited, I asked my fiance if they could stand up for him, wearing black dresses with red sashes that match my dress and the bridesmaids. He`s extremely upset now and now I have his mother on my case telling me I`m alienating the family and there`s been nothing but hurt feeling since I came along. What do I do? I have eight bridesmaids right now and my fiance has two groomsmen. I have it in my head that the bridesmaids walk with the groomsmen/groomspeople down the isle and he won`t budge on this. The wedding is far from traditional, but he won`t even allow his femal friend of several years to stand up for him? I am feeling very upset right now, almost to the point where I want to call off the whole thing so as not to deal with him or his family anymore. Please Help!!