You were wrong to assume, never assume anything about weddings.
However, I lay the blame squarely at your brother's feet for the upset. IME of wedding grooms (I've attended many close friends and relatives weddings), bar choosing their suit they pretty much show up for the day and that's their contribution to the wedding. However, he should have known and communicated details about the wedding to you all when your mum began asking. He should not be saying I'll ask wife, he is getting married too. It is his job to advocate for his family.
I know I had a coffee with my MIL and GPIL's and told them wedding plans, meal plans (they were pretty much giving themselves hernias about the food), seating, anyone who we have missed off ect as my DH was rubbish at communicating and INVITING with his family and they were getting frustrated. I had asked DH to tell them, I sent the menu with him to give to them and it came back crumpled in the bottom of his rucksack, and ask certain questions but he "forgot" or only told them half because he CBA with the drama/ questions/ family gossip. DH drew up his list for his family. After chatting with MIL and GPIL there were aunts missing, cousins missing, close family friends missing. I only asked as i'd got 60 on my list and DH had 15 including himself and BIL who was best man. We had a package that allowed us upto 80 day guests and 120 evening guests, with a very reasonable per head extra charge for over 80 day guests.
I have a sneaking feeling that this was supposed to be a child free wedding and it was up to DB to tell you this, but he hasn't. Regarding the flower girl dresses, I imagine there were originally no flower girls (only wedding I attended with a flower girl, the flower girl was the bride and grooms 3yr old daughter). But your DM has gone on and on about it and so bride has caved and picked 3 people, hence why she's said don't bother paying for them.
It's going to be a matter of turning up and making the best of the day.