I'm really struggling to understand the opinions here.
If what the bride wants is a perfect-looking day with matching people and perfect photographs then perhaps she should hire models or cardboard cut out stand ins.
If on the other hand what she wants is for the most special people in her life to stand up and celebrate the marriage with her, then what difference does it make what colour their hair is?
What's more important? Photographs or people? The spirit of the wedding day or the look of it? What's the purpose of wedding photographs anyway? To be able to look back and think "What a wonderful day with all the people closest to me" or "Didn't my wedding look fantastic?" I hope to heaven it's the former.
I also don't understand the 'upstaging' argument. If a bride is so insecure that she thinks she can be upstaged on her big day by a *haircut*, when everyone has come to see her wedding, then... wow.
OP, it's your hair. Wear it how you want.
This argument is often given here, especially the cardboard cutouts or actors bit. It always seems to imply that anyone who wants their wedding to be a certain way - especially the look of their bridal party - is some kind of shallow dictator who only cares about putting on a show.
Well, all weddings are about a show to a certain extent. Of course they're about the ceremony of commitment, but if that was the only thing anyone cared about, why does anyone dress up? Why do we have wedding dresses, and fancy cake, and bouquets and flowers arrangements and special cars?
Most people ask their guests to adhere to a dress code. If we were only allowed to care about the spirit of the day and not the look of it, we'd be happy if everyone turned up in their pyjama bottoms if that's how they were happiest. It's a pretty rare wedding that is set up like that.
If we were only allowed to care about people, and not photographs, it wouldn't be OK to drag your bridal party down to the beach and ask them pose for photos. It wouldn't be OK to have your wedding on the beach at sunset because you like that look, because Great Aunt Mary doesn't really like sand between her toes, and we care more about people than looks, right?
People spend a lot of time, money and effort making their wedding look a certain way, yes, partly for the photos. As to the bolded, those two concepts aren't mutually exclusive. I look at my wedding photos and think both of those things.
Being in the bridal party, especially in Australia where the OP is, means matching dresses, hairstyles, and often jewellery and makeup. It's a look, just like asking everyone to dress formally. I wouldn't accept the job if I didn't want to do that.
We don't know why the OP's sister asked her to be a bridesmaid, but she did it under the assumption that the OP would look substantially like she did at the time of asking, especially given the update that the OP hasn't had a haircut in 5 years. She's not a completely shallow person who only cares about her photos because she wants her wedding to look like the vast majority of weddings she has ever been to ie. matching bridal party.
And as many have said - it's not about being upstaged by a haircut. It's about the stir it would cause if your normally conservative-looking sister had a radically different look just before your wedding. Regardless of how much the OP intends the new style to be just for her, the bride and others are going to assume that's she's doing it to make a statement. It's the statement, and not the cut, that has the potential to upstage the event.
Does the OP want her sister to spend the day of her wedding telling people that, no, sister and I aren't fighting - she didn't do her hair like that to annoy me, etc etc. And of course people shouldn't ask/gossip, but some will, and it might just take some of the shine off for the bride. All of which is avoidable if the OP just waits 6 weeks to get her hair done.