AIBU for refusing to straighten my kids hair for my brothers wedding?

This. I can understand hair needs to be styled and presentable for photos but straight is not the only way.

She is not the children’s auntie, she’s just a person that the brother has made a poor decision to marry

Yanbu your sil to be is blatantly racist

It’s not necessarily about something as explicit as that. It’s about insidious racist assumptions and beliefs that Caucasian hair is neater/more appropriate and Black hair is less so and therefore less acceptable. Going to deeper prejudices, it’s about what’s perceived to be civilised versus what’s perceived not to be.

There have been well-documented cases of workplace discrimination over policies requiring ‘neat’ and ‘professional’ hair with non-white employees being told that their natural hair (e.g. an Afro) doesn’t fit with this. Those workplaces have generally had to back down, I believe, but the attitude persists.

Why not copy and paste the message dsis sent sil changing the names obviously..see what response YOU get

I would go that far too. You can’t really skirt around the issue now OP. You have written evidence blonde curls are ok and brown/black curls are not. There’s no arguing whether this is racist an you have to defend your children here and call her out on this

She’s being racist and I would definitely use that word.

In light of this message I'd be sending her something along the lines of the following -

"I've spoken to my sister who said you have no problem with niece wearing her hair with her natural curls. That makes it pretty obvious that your issue with my children's hair because is their texture. We won't be whitewashing our kids or attending the wedding of someone so racist so please feel free to collect the dresses and they are no longer needed."

I absolutely think OP should go this far. Her future SIL is vile

'Dear SIL to be, at what point do curls stop being 'lovely' and become 'intolerable'?'

I think that message suggestion is far too polite!

With your DSis blessing, i would screenshot your sister's message and the bride's response and forward the screenshot to the bride with something like:

"...So it's just the black curls that you won't 'tolerate'? Wow! Shame on you and your racist request. I will be speaking to my DBro and deciding if we will still attend the wedding - if we do, it will be for him - but beyond that you have made your worldview super clear and I won't 'tolerate' racism towards my DC. We have seen who you are and won't be engaging with you in the future, so rest assured your future is white and straight"

I mean - you might not want to go that far but i am fucking raging on your behalf and I genuinely WOULD go that far.

YANBU to refuse to straighten their hair. But I'm not sure why their "understanding" has any bearing on how unhealthy it is for their hair.
I'd set up a group WhatsApp with her and your brother, and just say that you're very sorry if she feels that your children's hair will detract from their big day, but that you're sure people will still manage to work out that she's the bride, despite your child's curls.

Ask her if white people with naturally curly hair will be "tolerated". Or, indeed, white people with straight hair who have curled / waved it.

I would not attend the wedding of a racist person. You have a black husband and mixed race children, these situations will unfortunately come up at times and this will not be the only time you will need to defend your children and your family. I hope you make the right decision.

What should she say sorry? And it’s not cheeky it’s fucking racism!

I am so sorry you are dealing with this. Your future current sister in law is showing who you she is

Speak to your brother about it if it can’t be resolved take the kids to a theme park or something for the day instead I’m sure they would have more fun. They’re children for god sake. It’s sad that you would miss the wedding but if she won’t tolerate it that’s her choice it’s not rude to say no to that she’s using the suggestion that your being rude to force you to comply with her request.
Don’t do it they should never be forced to change there appearance to fit someone else’s opinion of what they should be.
my kids are white with feral curls I absolutely wouldn’t do it either for any reason they are beautiful as they are.

Yes I’d skip this wedding and wait for the next one.

After your latest update OP, I would screenshot that, send it to bride with a 'wtf, have a nice life you deeply unpleasant woman' then do something really nice with your kids on the day as a distraction and then have nothing more to do with her. It would be hill I died on. She's certainly not worth headspace in life.

@CrypticClueless Flat ironing does cause damage to the hair strand,

Literally what I said.

so it could take 4 years for the children to grow back undamaged hair from the undamaged follicle.

Hair grows 6 inches per year. The only situation in which it would take 4 years to cut off damage would be if you damaged hair strands at the very base, and you have your hair more than 24 inches long.

What An entitled bitch.

Stick to your position and reconsider whether you want this person in your children’s lives.

And I would also avoid the wedding and tell everyone why.

Ah I see the update - there you have it. Obvious racism. Call it out - don’t let this happen.

Wow. I wouldn't have my kids around her. Take them for a lovely day out instead