A few things come to me about this scenario:

*Since the 'bride' has 4 children, has she been married before? Maybe she was 'ram-rodded' the first go-round and vows 'this time it's her way or the highway?' Either way, she has a lot more 'life experience' now and knows what she wants. Period. Her wedding, her day, her way. If your brother is o.k. with it--you guys should butt out. If he isn't o.k. with it, still butt out. He's obviously a big boy and he can either speak up for himself or live with it I suppose. He will be the one that has to live with her in the end. Maybe she's wonderful, maybe she's the evil one, but you guys are setting the stage for a very rocky relationship with your soon to be sister-in-law, etc. Don't go there. It will end badly I promise.

*It is possible that she was burned majorly by her kids' father's family? This would explain a lot.

*If your family isn't contributing monetarily to the wedding, maybe she feels you don't want to truly be a part of the wedding? The old, put your money where your mouth is scenario? (I'm another person that the inlaws didn't contribute one dollar to our wedding--THEIR SON'S wedding--yet they thought they could contribute their opinions, control the guest list, insist that DH's sister's spoiled child be our flower girl and purchase her a dress before we even decided on her or the dress, get all of our wedding photos, etc.). I'm of the belief that, hey, in this day and age, both sets of parents should be contributing or the one that isn't needs to back off and out of it. I mean, it's their SON'S wedding just like it someone's DAUGHTER'S wedding. I am of the belief that you can't sit back and expect the bride's family to pay for all and still dictate. Offer to help financially in some way. Weddings are very expensive and for most everyone entering marriage, they are thinking this is a one time shot at getting it 'right' and 'their way' (maybe the bride feels this is her very 'last shot'). You can't tell others how to spend their money or who to have participate in their party. I'm NOT saying this is your family or how they are behaving at all, but maybe the bride has 'been there done that' ya know in the past? A lot of people still believe very traditionally about this--generally the bride's family covers the wedding and she choses all the attendants, etc., so maybe that is where she is coming from? I'd say if your brother is happy with it, let it be. It's a hill to die on. You don't get a vote. I promise, your big day will come and you will feel the same way I'm sure. You will not appreciate any and everyone inserting themselves.

*Could it be that the reason the bride is relying on HER side of the family for catering, etc., is because she knows 100% that these people will come through for her? It's a crap shoot with everyone else as she doesn't really know them at all or well. Again, nothing against your side of the family, but maybe that's how she sees it. Peace of mind maybe?

*Maybe they only have a large enough budget to include her children in the wedding party and then they obviously need escorts? Who knows. Could it be that these children requested their cousins as escorts rather than people they don't know very well? Kids are like that, especially shy ones. Maybe it was, "Alright, I'll do it, but only if xyz walks with me"?

Now, I do understand that you are relatively the same age as the other parties in the wedding, and I can feel for you that your feelings have been hurt. I do understand that and I am sorry. Right off I can see that it's easy to feel slighted like that. But honestly, I'd say unless you are good friends with the bride or the children of the bride then I don't think this is meant to be a slap in the face to you or your family. I apologize if I sound harsh in any of my words, I'm not trying to be like that at all (it kinda lights my fire though as I have dealt with the inlaws in a similar situation).

Here is what I would do if you guys hope to have any sort of relationship with your soon to be SIL in the future (And ultimately this will include your brother, because if you end up alienating your SIL, you aren't gonna be seeing much of the brother either likely or any future children entering the picture. He'll rightfully choose his new wife.): Get over this quickly and quietly. Let it go. Don't talk to your brother about it, it's going to get back to the wife and she'll remember it forever. No one deserves this sort of black cloud hanging over their big day. It WILL affect all future dealings with them, I promise. Get together with your family and throw them a very nice wedding shower, even if it's just for your side of the family (throw the major one if no one else is doing it--what I'm saying is don't overstep boundaries if someone else is already planning it). If that won't work, do a new husband shower (tools, bar-b-que, etc.) and do something special for the bride (a massage, salon trip, gift card, etc., don't leave her out is all). Offer to do something special for the honeymoon. This could be various things, money, a special gift basket to ride alongside them in the 'getaway car', help take care of their children (or check on them/do something fun with them since they are older), etc. Be genuinely happy for them and let them know it. You can always ask her if there is anything at all you guys can do to make her special day more special. Just be prepared to let it go (w/out taking it personally) if she declines. HTH.