I have mixed feelings about this.
I think if it's a friend's wedding, then you have no right to be offended if they do not want to invite your children. Many of my friends barely no my children and I certainly don't expect them to want them at their wedding.
I do think family is different though. If the children are an integral part of your family, they should be there.
There is a danger of Bridezilla taking over in the 'it's my day, my choice' attitude. Yes, it is your wedding and you have every right to do it as you wish, but you have to accept that, in being so dogmatic, some people may not want or be able to come.
and if your brother being there is less important than his children not being there, then at least you know where you stand!
Talk to MIL, explain you are not happy leaving your 3 children with someone else for so long, then don't go.
Their wedding, their choice, they can't complain if you can't go.