What kind of relationship did your kids have with your husband's brother or his children?  I would have been more on the annoyed side if it was my children's step siblings that were excluding them... but it is his brother's family. 

Unfortunately, weddings can be very expensive to host and the bride and groom sometimes have to draw lines as to who to invite.  If they didn't know your children well.. perhaps not inviting kids in general(the 12.5 yo would fall here) or adults who they don't know well.. (your older son).  It sounds like your DH's other kids are excluded as well.

I don't think it really is meant as a slight more like that they don't know your kids well and had to limit guest size. 

However, if you don't feel you can attend as a gracious guest, you can rsvp your regrets.

Personally, I am not the "invite everyone who is related in any form" kind of person. As ESMOD said, weddings are expensive, and I would personally only invite people I have an actual relationship with. For me, this even includes my own cousins that I didn't grow up having a relationship with. I wouldn't want my wedding to be aquaintances, I would want it to be with people I love and am close with. I wouldn't do this to be rude, it is just a personal preference. I'm assuming your DH's brother's kids weren't that close to your kids, and don't have a relationship outside of family events? I wouldn't take this as a slight to your kids, I would just assume that the bride and groom prefer to have people they are close with there.

I think there are two trains of thought when it comes to inviting wedding guests: Those who believe every family member, close or not, should be invited, and those who believe only family and friends that are close to them should be invited. I don't think either is wrong, although I don't think anyone should feel pressured to invite guests simply because they are a relation of sorts.

I didn't invite most of my cousins to our wedding and the same went for my DH. Had we invited every single cousin, step cousin, and their families we would have been paying for another 50+ plates of very expensive meals. The final count for the entire guest list was 72. 

I highly doubt your own kids will invite every single cousin of theirs to their own weddings. 

I think a spouse or SO of a gues must be invited.   Other than that, optional.  I am guessing that the stepkids did not grow up together.  

I think you should go if you want to, but not going becuase your kids are not invited is a mistake.  Please do not nag anyone that your kids deserve an invtie. 

Personally I wouldn't expect my children to be invited to their step-cousins's weddings or any of their private events. The only time they've hung out has been at a handful of holiday gatherings at MIL's. Are your kids feeling upset about this or just you?  

I know you feel hurt by the lack of invites for your children or two of your stepchildren.
Your feelings are genuine and should be acknowledged.
I do not mean to offend you by writing this, but I not see cause for you to feel aggrieved.
There may be other underlying issues within your family that make you uneasy, but even then, compaints about kids and invitations are going to be brushed aside by most people.

It is my belief - you do not have to agree - that no is owed an invitation to anything.
It is the right of the host to decide the guest list.
This may hurt or offend you, but I am sure that is not the primary intention of the host.

As others have mentioned, large functions are expensive.
Hosts generally want people there with whom they share a personal connection or relationship.
Your children - and here I mean one's children, not specifically your own children - may not meet that criteria for the host.

I am saying this in general, and not only to you:
I don't know how many times, I have heard people complain about their own children being excluded to a family event - especially weddings.
It is taken as a personal slight.
Some people specifically do not want your children there.
They have their reasons.
Some people do not want any children there.
Some people will make an exception and have the children of immediate siblings or close friends present.
If you aren't immediate sibling, do not complain.
If you do not have a close relationship with the host, what right do you have to complain?

An invitation means you can accept or decline.
If you do not want to go, then decline.
Being ungracious just means the invitations are fewer.

I would not expect an invitation to a step-cousin's wedding.  I certainly would not be offended if I didn't get one.

I'm getting married next year.  I'm not inviting cousins, let alone step cousins.  I think your expectations are unreasonable.

If you've been together ten years, and married six, and they have never invited your children, I don't see why they would now. They don't even invite two of your husband's children. They have made it clear that they don't consider your children family.

Their only saving grace in this is that they invite you. They actually acknowledge the marriage. There's no BM is still family so she's coming and not you.

Look on the bright side. When your children get married there is no way they have to be invited. You'll save money by not inviting them.

My advice is to let it go. Attend with DH, dress like Sophia Vegara, and smile your way through the day.