Managing family without being a bridezilla?

I have a problem. My sister is two years older than me, and happily lives alone with her dog. She has never shown any interest in having a romantic partner, which is absolutely fine. However, any time we discuss my upcoming wedding (and her role as a bridesmaid) she says something like “this is the closest I’ll get to having a wedding” or “I’ll never get married, let me pay for X”, usually something odd like my dress, which I already bought myself. Big sister and I did not get on as kids, and sort of affectionately respect each other as adults, however she does have a habit of making everything about herself. I’m the classic middle child, and often felt our parents sort of left me to get on with things, because I’ve always been rather self sufficient, and with multiple kids, he who shouts loudest gets parental resource. Sister always needs some kind of special treatment - for example, she wanted to come home for Christmas (3 hour drive) but didn’t have a car yet. My Dad did a six hour round trip to get her, and again to take her home. I’d have not even asked, and just taken the train. Everything I’m planning - from the dresses to the seating chart, to the accommodation, consciously revolves around what might make her the least likely to complain.

She is VERY excited about being a bridesmaid. And I’ll be honest, I sort of just wanted to ask my lifelong best friend. But sister continually asked me, from the moment I got engaged (even dropping hints years before, which honestly shocked me because I didn’t think she’d be bothered in the slightest) and I think leant on our mum to nudge me also. So I gave in and asked her. But here’s the thing - she won’t do anything that she doesn’t already want to do. For example, I’m having a really hard time finding a dress to match our colour scheme that will allow her to wear a proper bra while also suiting her shape and skin tone, because she absolutely refuses to even try a strapless one. “I’ve got a flesh coloured one, it’ll be fine”. And the mid to end sections of her long hair are currently bright blue, but may well be something else entirely for the wedding. Basically, she wants all the perks of being a bridesmaid and being “in” the wedding without actually having to do anything.

Up til now I honestly think I’ve been the most chill bride. My best friend wanted to organise my Hen, I set up a WhatsApp group with all the girls in it and left her one rule - Do something simple that everyone can afford. I’ve shopped around and bought my own dress. I’ve booked and paid for hair and makeup for myself and both bridesmaids, and offered the same to both mums as a courtesy. I assembled my invitations myself, I am not roping people in to make table decorations or favours. I’m giving my parents nothing to do so that they can experience the day as a guest, not someone with a job to do. I feel like I’m constantly badgered by my family to let them “help”, but they won’t do the tiniest thing if it’s even a little bit inconvenient. My mum is the same, their excitement is manifesting into a stream of unwelcome “suggestions” and I just don’t know how to push back without being accused of being a bridezilla or as my mum would say, “difficult”.
I’m really not like “you must all weigh the same and dye your hair this exact shade of brown” militant but in contrast, my best friend says “tell me what you need. Tell me where to stand and what you need me to wear, I’ll wear a chicken on my head if I have to”. It just really highlights how vastly unhelpful everyone else is being. I just need them to get on board or leave me alone!
How can I bring this up without them getting immediately defensive and causing an argument?
NB: I have not asked anybody to wear a chicken on their head.