You and your mother were unreasonable to assume your daughters would be flower girls
It sounds like your mum has overstepped a bit by trying to involve herself in the planning multiple times
I think your brother has been unreasonable not to tell you that your children aren't invited sooner
I also think your brother and FSIL are unreasonable to invite some 'family' children and not others- it's their choice, but a it's a sure fire way to upset people and doesn't help to foster good family relations
Your step dad is unreasonable not to go (I'm a little unclear, but think he has been invited)
There's not enough info to know to comment on the lack of invite for your step siblings
All said, I can understand why you are upset OP. Unfortunately, unless you want to distance yourselves from your DB and FSIL, you and your mother need to keep quiet and act like the perfect guests. However, when asked where your children are (which will happen, unless this is a tiny wedding), I would have no issue stating clearly and factually that they weren't invited. I would also not be putting myself out if asked to help with this wedding at any point