help wedding guest outfit

[AmericasTorturedBrow]

my DBro is getting married in September to one of my good friends (yay!) but I have no idea what to wear.

It's a very relaxed wedding in theory, fairylights and country wildflowers, they've stipulated no dress code but I know they want it all relaxed and English country garden style.

Trouble is in practise it's fairly formal - I haven't seen her dress but know it's a big name brand (not gypsy princess style but I think it's will be fairly wow, she's bloody gorgeous so would make a bin bag look frankly amazing), my DBro is in the army so will be in his formal wear, her sister is maid of honour and last I heard is wearing a floor length navy dress, my DS will be a sword bearer (DD isn't in wedding party but will be nearly 4 so will be in a pretty frock). Venue is a grand country house.

Does it matter what style I go for? Was thinking of wearing a knee length peach summery/slightly formal dress I have but worry that if everyone else is in blue (my mum is wearing royal blue) and considering the setting I'll look out of place weird in photos....does this even matter? DBro and SIL insisting to wear whatever I want. There's a chance I might end up stepping in for DBro's Best Man who may not be able to make it

Ugh, so what kind of style do you all think I should go for? long and more formal and in blue colour palette, or just stick to my more summery dress?

I would go for a more flowery patterned, summery colour dress but in a classic, formal cut. Which kind of sounds like you are describing! So if I was buying new for it, I'd go for something like this or this.

hmmm interesting, I hadn't even thought about flowery patterned dresses, the right one could def do crossover of country wedding and posh english house....wedding is happening in an orangery and dinner and dancing is in a big old hall FYI (complete with minstrels balconey etc)...they're having a gin bar in the evening and "breakout" rooms are generally heavy wood, big open fireplaces etc, but they're hoping a lot of mingling will happen in the gorgeous gardens...this is so hard to dress for!

Those dresses are both v pretty and occasion-appropriate.
I suppose the main question is whether you will feel grumpy if you decide that you 'have' to buy a new outfit. For my part, the word 'peach' makes me shudder and I would regard my brother's wedding as absolutely deserving of a sartorial splurge; but if there were financial implications then I'd sod the photographs and wear an outfit I liked and felt comfortable in. Which I think is what your brother and future sister-in-law are trying to tell you.
Flowery is a good choice, esp with full skirt for ease of dancing; you can also never go wrong with a lace shift, with sleeves if you're worried about arms.

ok cool - the dress is I guess more coral than peach, and I do have shoes that go with it which helps. Cost is only a factor because I have a couple of dresses bought for weddings in the last few years that haven't seen the light of day since (both way more formal and very specific dress codes) and our family is travelling across the globe for the wedding....but floral isn't something I'd thought of at all so thanks for that new train of thought, will get thinking on that one for sure