Your update gives good clarification.
@TibetanCherryTree
your eldest isn’t his niece
Yes she is. She is his Step-Niece
Step is a very significant word here. If your stepmum isn't your mum, why does your niece suddenly forcefully become a niece. Not many mums would want a stepmum called a mum.
I have 2 DDs. One has previously been asked to be a flower girl and the other hasn't.
It's not my right to demand that both my DDs are included in the bridal party. That's way too entitled.
It wasn't a child free wedding, so DD1 was still invited. There are 2 years between my DDs and the older one wasn't upset about not being a flower girl at all.
I remember the same thing as a child. Sometimes Dsis was a flower girl and I wasn't...and vice versa.
I personally wouldn't want to not invite DD1 in the OPs case and they could have made an exception even if it was a childfree wedding. However, it's very clear they want a total childfree wedding, excluding the bridal party.
Childfree weddings usually allow for children of the family to attend...so in that respect they are being unreasonable.... but if your DD got to come, the cousin's parents could get upset that their kids were overlooked above a stepchild.
I think not having her as a flower girl is acceptable tbh.
I chose one niece in law to be a flower girl at my wedding and not her sister. I had my own niece as well and didn't want 3 flower girls. My BIL began moaning to DH about it.
I pointed out that the bridal party was my choice, as the grooms party were his choice. Perhaps your MIL should not have intervened in this case.
Life isn't always fair and I think parents very often make these situations worse by projecting their own feelings. Maybe some form of guilt that the child is part of a blended family following a relationship breakdown.
The reality is she isn't exactly the same relation to them as their own GD.
This is not the same with an adopted child as pp have tried to compare...because an adopted child doesn't go to their dad's EOW. If the couple split, grandparents can still see the adopted child. They have no right or entitlement to see DD1 if you split up with your H.
Has anyone ever thought that the second child in these scenarios could feel miffed about their half sibling getting more presents than them. I've got a colleague who said she resented having to 'share' her grandparents and seeing her half and step siblings come back from their dad's houses with gifts from dad and paternal GPs.