Incidentally, my husband and I got married in a registry office with strangers as witnesses (we had a ceremony and party 8 months later), therefore, I didn't really feel that it would be such a big deal if he didn't go.
But that was your choice. You made a decision so you can enforce a similar decision on someone else? I don't think so. How unpleasant.
You're being nasty and very selfish, actually. You and your children haven't replaced his older children. And if things haven't always been great on that front, then his son his probably trying to best meet everyone's needs whilst also taking this opportunity to build the relationship with his dad.
I agree with whoever said that this will be about appeasing his son's mother as much as anything. Having been in that position, I can only imagine the handwringing that has gone in to making the final decision. And, of course, there's always someone who isn't happy because they think the decision should have been all about them. (When it is probably nothing to do with them at all.)
I think it would be really petty and churlish of you to not attend. Unfortunately, you need to suck it up and behave like the big grown up person that you are. You are not the most important person here.
Oh, and if your step sons show an interest in building a relationship with your children; their siblings, then I would encourage and welcome that with open arms.