<human>: She looks at this board so throwaway.

My sister got engaged a couple months ago. I was happy for her and things were fine until she told me I was going to be a bridesmaid. 

A little backstory: I hate weddings. I've never enjoyed myself at one and I always find myself wanting to leave. I just don't want to be involved or wear a dress I'll only wear once. My sister knows this all too well. 

I told her thank you, but I would have to pass. She cut me off saying that I had to be one because I was her sister. I emphasized again that my answer was no but she acted as if I hadn't said anything.

I told my mom about this but she thinks I should just do it for my sister. I'd rather not be the grumpy bridesmaid at her wedding and that she ask someone who would be more excited to be in the party. Am I being selfish?

Can you condense the text into a few key points?
<bot>: sister wants me in her bridal party. i declined but she says i'm obligated to be because i'm her sister