My sister is moh but won't do the minimal tasks

My sister and I are close. She’s my only sibling. I asked her to be my MOH against the advice of my mom who said she wouldn’t be good for the part and not to expect much. I didnt expect her to be someone who would be super excited about the wedding but I still wanted her to be my MOH. My wedding is in 4 months. She has asked one of my bridesmaids (who is like my sister from another mother) to do the speech because she doesn't feel comfortable talking in front of people. I said it was fine. Now, She doesn’t like the color I picked for the dress so she still hasn’t ordered it. The color she wants will not match in my opinion and will stick out like a sore thumb. I offered other style options, she doesn't like any of them. Now she also told me that she doesn’t want to stand during the ceremony, and would rather sit. She also has complained many times about taking photos and everytime I tried to accommodate her, her response is “Im going to feel miserable anyway so It doesnt matter”. Im starting to feel like maybe she said yes to being MOH so I wouldn't feel bad. In reality I want her to have a good time and wouldn't feel bad if she wasn't MOH. and when I explained this to her she said “i cant talk about this right now Im busy” At this point Im wondering what would be the point of her being MOH if she wont be doing any of the basic MOH things MOHs usually do, ex. wearing the dress I picked, standing next to me, and doing a speech. What would be the point of her wearing a different dress signifying her as the MOH?? Am I wrong here?