Have them both stay home. They arent pawns in a favourtism game. Weddings are boring especially for kids. And some adult!

That is not the slightest bit unreasonable. It is their day, their wedding and they can out whom the like in there. There should never be any expectation that everyone is included. And how wonderful they want photos with all the kids.

May be an unpopular opinion but I wouldn't do that to my kids either, especially since they're so close in age and your daughter is too young not to just feel inconsolable and excluded. It would definitely feel like her brother is the favourite.
Both or none in my opinion.

This is so petty. Your 4 year old need to wise up, and quite frankly - so do you.
Tell your daughter to suck it up. Shes old enough to know better and once she see's how boring that whole affair is, she'll be fine. Kids hate ceremonies, they're boring - she'll probably be glad she wasn't involved once she see's the boys are having any fun.
Straight up though, you seem proper bitchy about it. Suck it up - it's not your wedding.

As a wedding celebrant, I find any child under about 5 can be problematic. Some are absolutely wonderful and others last about two seconds before running for Mum in tears. It makes life interesting because I never know which way it's going to go beforehand. I do not let it disrupt the wedding completely and most people are pretty sensible. In the end, it's the bride's decision who to include.

This is your husbands brother what does hubby have to say about all of this?

No unreasonable. I would do the same as you.. I don't play favourites with my kids and will not allow anyone else to.

Your choice, but if it had been your daughter who was asked and not your son, would you do the same??? If one child gets asked to a birthday party do you say no because the other wasn't??? I would still let my child if they were asked, (but that is me). You can't force the groom or bride to let your child be a flower girl that is the brides decision and she might have already picked her flower girl. What is she suppose to do, break her heart to make you happy??? I understand where you are coming from. I hope you didn't tell them no unless you let my daughter be the flower girl, that would sure start the newly weds in fighting before they get married. When I got married I allowed children at the Church but not my reception, as I just didn't want kids there. I organised a baby sitter for those who needed one after the church ceremony. If you feel you have done the right thing, then it is the right thing for you and your kids. Just don't make a big thing out of it. Just explain to them how you feel and leave it at that.

Yes it is unreasonable. And I think your being a bit dramatic. Really how devastated will a 4yr old be. Just distract her or make something up or simply explain on the day if it’s a problem that that is just how it is. Kids need to learn resilience these days sometimes it’s ok for one to get included in something and one not. Why would you not take them to the wedding? That’s is your husbands brother doesn’t get much closer than that. You should be honoured he asked your son who is only 6 to be a part of this special day. His future wife who is no relation to any of you has no obligation to ask your daughter to be involved

Wow that’s a dramatic reaction! As far as I know the grooms choose “their” people for the bridal party, the bride chooses her side. They only ever mix sides if there’s a numbers problem or lack of family to fill spots. But the bride is not obliged to put your daughter if she already has nieces or a god daughter etc
Just put miss 4 in a cute frilly dress with a nice floral head piece. Make her feel like a princess for the day without creating all this family friction because your 4 yo is supposedly gonna have a meltdown 🤦🏻‍♀️

Is it worth all the drama it's going to be to go against the family? It's one day. If you explained to her i'm sure she'll understand my 4 year old boy would & let her know she'll be getting her own special reward for being such good sport.

They just want them there for photos? No you’re not being unreasonable at all.