I think that is a recipe for disaster, personally. That is exactly how you escalate a situation into outright conflict, which seldom gets anyone what they want.
It’s absolutely not up to OP or her mother to dictate what is or is “not on” with regards to this or any other wedding. That level of entitlement is absurd. The bride and groom decide the guest list to their own wedding (which they are paying for).
They may well have their own reasons for the decisions they have made- it could be that there are number restrictions at the venue or cost restrictions. Or perhaps OP’s DB has a very different view to OP with regards to his relationships with his sister, or his stepfather, and/or step-father’s children. OP might be close to her step-father/his children, but it is not necessarily true that her brother feels the same. Or perhaps he is just fairly distant from the whole planning aspect.
The couple wanted a child-free wedding, excluding the children in the bridal party- it’s not exactly unheard of or unusual. It might not be your (or OP’s) preference, but it’s not your wedding and so you should respect that other people don’t share your views in everything. If OP cannot do that, chooses to make it a personal slight against her daughters (which it is probably not), then that’s up to her. I have children, I really don’t see why it is an absolute necessity that OP’s children are invited? Why is it a source of such outrage?
However, if OP really can’t see past it (or has a childcare problem), then she’ll just have to decline the invite. If asked why, she can explain. What you don’t do is wade in, make demands and issue ultimatums with regards to other people’s wedding (even a family member) and expect everything to go smoothly thereafter.
as for “it’s terrible the brides cousin’s children are flower girls but the grooms nieces are not”- why? They may be closer genetic relatives to the groom than the brides choice, but it seems very possible that the bride is very close to her cousin and her children. Degree of relation by blood is not the only factor that matters. The bride is free to choose her bridesmaids and flower girls, that is absolutely normal. Perhaps OP and her mum should also get to decide the MOH and best man? If not, why do they get a say on her choice of flower girls?