Is it wrong of me to decline to be a bridesmaid?

I was a very reluctant maid of honor when my sister got married. The only reason i accepted the role is because my mom and sister threatened to disown me if I don't say yes. It's not really because of all the stuff that come with being a bridesmaid that I don't like. I'm just not a dress type of girl. When I attend a wedding I wear dress suits. I could probably name the last 10 times I've worn a dress and that date back all the way back to elementary.

When my sister got married, there was 4 of my cousins who were bridesmaid. I'm a size 10, I don't think of myself as fat, but I know I could slim down a little. What sucks is, 3 of my cousins are size 0 and 2. If it wasn't for my other cousin who is the same size and height as me, it would make me feel so insecure.

My brother and his fiancee are getting married this June, and his fiancee has ask me to be one of the bridesmaid. She is half Cambodian, and in her culture the wedding last 2 days. Not only would I have to wear an evening gown, but the first day of the ceremony I have to wear a traditional Cambodian bridesmaid's gown. I turned it down, which she was disappointed, but she didn't push it. My brother was more pushy, when I explain I just don't like to wear dress, let alone dresses. He said what difference does it make you'll still attend my wedding in a dress. That is when I explain to him my plans of wearing pants.

Some of my friends tells me I shouldn't have to wear a dress if I don't feel comfortable, but I also have friends that tells me it would be such an experience to be able to wear another cultures outfit.

 Should I just bare it out and wear the outfit or is it okay for me to decline being a bridesmaid?