I’m really surprised by the replies here supporting OP. You’re entitled to your feelings, but it doesn’t mean your DB and SIL are in the wrong either.
Unless your DB and/or SIL have a habit of being deliberately unkind to you/your mum or your stepfather, then I think you need to step back a bit and think about why what you want is more important than the choices they have made? None of these things- your SIL choosing her own wedding party, nor them wanting a child-free wedding, or not inviting adult step-siblings, is necessarily unreasonable. There may be good reasons for the latter two factors and your SIL is absolutely free to choose her own bridesmaids and flower girls, so has nothing to apologise to you for.
I think you are absolutely unreasonable to assume that your DD’s were going to be flower girls. That you continued to make this assumption when no request from SIL has been made is even less understandable. I imagine your DB and SIL had no inkling that you and your mum would immediately jump to that conclusion, so couldn’t pre-empt or advise you otherwise because it hadn’t entered their heads that you would do so! I don’t understand why you did either, it’s really absurd- and totally ridiculous to be annoyed or upset that your assumption was incorrect. Firstly, not every bride has flower girls; and secondly, you clearly aren’t that close to her as you don’t seem to know much about her, nor seem to be willing to actually talk to her directly. Your SIL probably chose her bridal party because of her relationship with them, as is exactly her right. Or perhaps she has been pressured into asking the cousin’s DDs to be flower girls by her own family? Or she really wants her cousin there but as they live in Ireland they could only come if the children came too? Who knows- OP doesn’t seem to. I would also say that it is really not your place to judge her family relationships- if it is the case that she is close to her cousin and her children, who are you to decide that it’s “not close enough” or that she should feel closer to your DD’s than her cousins children? It’s more than a little presumptuous. You can be upset, but I think you should acknowledge that your DB/SIL have not done anything wrong and you put yourself in this position.
As for the decision not to have children at the wedding (other than flower girls)- that is hardly an unknown phenomenon. Lots of weddings are child free or child free other than the wedding party- it is not a slight to the children as individuals and I think it is daft to make it so. Sometimes it’s personal preference and sometimes it’s to do with financial restrictions/venue space- people often have several children and it can quickly add substantially to numbers and costs. Either way, their choice to make.
You don’t have to like it and it might not be what you chose to do, but it is not unreasonable because of that. Of course, the bride and groom have to accept (with good grace) that some guests can’t attend if there are childcare issues for those parents invited. You are entitled to your view on child-free weddings and can choose to be insulted by it if you want and can then choose not to attend if that is the case- I think it is slightly like cutting off your nose to spite your face. Your girls are young enough that they’ll only be really upset about it if you choose to make a fuss and tell them they should be- I hope you hadn’t told them they were going to be flower girls? I have children and really can’t get upset about child-free weddings. I don’t view it as a personal
insult to me or my children, nor a reflection on their views of my children as individuals- why would I? I think it is bordering on self-absorbed to think that it is.
with regards to your step-fathers children not being invited- again, there may be good reasons or this, or perhaps a thoughtless oversight. You may feel very close to your stepfather and step-siblings and that is a lovely thing for all of you. But does your brother view them the same way? He may like them all well enough and without any animosity, but does not feel close to your stepfather or his children. It’s entirely feasible that he does not feel about them exactly the same as you do. And if that is the case, what gives you the right to decide his feelings are invalid and yours are the right ones? Your stepfather is most likely a lovely man from what you have said, but your brothers relationship with him is probably different to yours and his experiences of him not the same as yours. After all, full siblings can have very different experiences and relationships with their own parents, so it’s possible that you and your stepfather have a great relationship but your brother’s relationship with him is more distant or strained. Or maybe he and your stepfather get on fine but he genuinely doesn’t get on with your step-siblings? Perhaps it’s a numbers thing- to invite them he would have to not invite someone he wants there more? Perhaps he isn’t much involved in the planning and forgot to get your SIL to include them? Unless he has form for being deliberately hurtful to your stepfather or his children, then I wouldn’t assume a deliberate attempt to be so until you have good reason to back it up. Have you asked him why?
The fact you and your DB were invited to your step-siblings weddings is a red herring- that was (presumably) their choice and it would have been churlish for him to turn down the invitation if he actually gets on with them/likes them. And if he doesn’t, I imagine he thought declining the invitation without good justification would cause problems?
I can understand your stepfather being a bit put out, but I don’t think It’s the worst thing not to invite adult step-siblings (that you met as a late teen/adult). Are your step-siblings upset at not being invited- if they are is it because they feel slighted or because they are upset on their father’s behalf (as he is obviously upset)?
As for your mum paying for the dresses- I think your SIL has given her an out there and clearly doesn’t expect your mum to pay given the letter she wrote. She is not “being cheeky” to send the bill- your mum insisted on paying for the dresses based on her erroneous assumption that her granddaughters would be flower girls. That error was all down to your mum, and your SIL has already said it is fine for her not to pay. So, it’s up to her to decide whether she does or not. You, your mum and your step-dad are totally wrong, in my opinion, to hold that against her. The problem was all down to you and your mum making unreasonable assumptions.
Reading some replies, I understand why people elect to elope. So many opinions and demands that it soon stops being about the couple getting married and more about appeasing family members views on what should happen, who should be there and what roles they should play. My friends family made her wedding more about themselves and she cancelled the wedding whilst she still could and eloped as it had made her so miserable. I was so sad that she felt the need to do that because her family (and future in laws, to be fair) were more interested in their own wants and views - and in a few cases fragile ego’s- that they made her so unhappy that she and her now husband felt the best thing was to cancel and elope. What really rubbed salt in the wounds was that several members of both families were very clearly insulted and/or upset that they had eloped. in her case, her mother and her own sister are the most vocal about it- and two of the biggest instigators of the problem. It has really affected their relationships. Her mother in law was also angry but did actually have the decency to apologise when her son told her exactly why it had happened.
At the end of the day OP, you got it wrong re the flower girl situation- own it. You are entitled not to agree with child-free weddings but have to respect that others have a different view. I wouldn’t take it as a personal slight against you or your DDs- and unless there are going to be many other children there and your children are being specifically excluded, then it almost certainly isn’t. You can choose not to attend if you are mortally insulted. Your stepfather can also choose to be insulted and not attend.