I am 100% with you here @Notmotherofflowergirls but I do think your mum was a little bit out order pushing for your 2 little girls to be flower girls.
I have to agree though, that your brother doesn't sound interested in the wedding, and seems to be distancing himself from it. Like most men. My 2 cousins daughters (both early 30s,) got married in 2022 and 2023, and they did 95% of EVERYTHING. The men did virtually fuck-all. Pretty typical really, women do fucking everything! 
I digress. I have 2 daughters (born mid-late 1990s,) and my younger brother (10 years younger,) got married when they were 5 and 6, and his wife and her mother were organising the wedding. And my SIL, her mum, AND my brother all asked me if my 2 DD would be bridesmaids. I said yes of course. I was chuffed and surprised that they asked. Even so, SIL had/has 2 nephews, so didn't have nieces on her side, but she could still have not bothered with mine and DH's daughters. She wanted my little girls as bridesmaids. They were so happy, and looked like little princesses. 👑
Sorry not helping!
I think YANBU at all, and have every right to feel hurt that your DDs are not being asked to be flower girls/bridesmaids. On what planet is it ONLY the bride's nieces/female relatives who are flower girls/bridesmaids?
My friend's brother got married in October last year, and her daughter (age 9) was a bridesmaid. Ditto my work colleague several years ago. Her brother got married, and her daughter (aged 11 then) was a bridesmaid. Ignore the people saying YABU and dramatic and attention-seeking. And yeah, it's a bit mean of them to not invite your stepdad's children.
I would go to the wedding, and take my 2 girls, but would give them all a wide berth afterwards. They don't appear to care about you and your lovely girls, as much as you care about them. Sorry. 
I am also sorry your mum is feeling left out of everything. No wonder some women want at least one daughter!
One thing confuses me, why does your brother's wife-to-be think your mum was going to pay for her cousin's children's dresses?