So your db and his fiancee are not able to invite all the biological aunts etc they want to their wedding.
They have invited his db (you), dsil, and 3 biological dn's and left out 2 step dn's.
I can understand why they have done that, they don't want to sacrifice more biological family members to add in step members.
But it has caused a problem for you and your family, as that is what they are to you, and you are now torn between your dw and 5 children, and your db and his fiancee. A really hard one. (I assume you want to go to be part of db's wedding)
There is probably no way to sort it out without someone being hurt/not liking your decision. I think you need to sort it out in such a way that you support the most 'valuable' people to you. If I or my dh were put in the same position we would just quietly decline the invitation. If a db didn't want all my family there, biological or not, we wouldn't go.
It would be different if none of your children had been invited but your db and fiancee, having invited some of your children, have caused a problem and they should be the ones 'put out' by you refusing to go rather than your wife feeling 'put out' by you accepting your step children are not part of the wider family by accepting the invitation.
There is the possibility of you going alone, or with your dw, leaving all dc's behind; or you offering to pay towards all 5 of your children going so db doesn't need to 'fork out' for them all, but I think the damage of finding out your dsc's are not necessarily viewed as part of the wider family has been done and can't be reversed. (They may be viewed as family, but not as high up in the hierarchy as biological aunts, uncles, nieces, nephews etc.) It's possible it will leave a sour taste in your dw's mouth and she will never look on your family the same again. She might also be hoping you don't think similarly to your brothers (hopefully you don't) and she may need a bit of tlc, extra love and a whole heap of support and reassurance from you.
I don't envy you your decision, but I know I would rather my dh supported me over his db, unless I was wrong.