I think you were a bit presumptuous by saying that your second child will be at a wedding at a young age. Sometimes kids aren't invited to events and it's rude to assume that they would be.
I would wait and see how this plays out since the wedding isn't until July. If you are asked to be in the wedding party, then you tell the bride that you'd love to be there, but you can't leave a one month old at home. If the baby can come, then fabulous!! We'll all be there! But if not, you'll have to turn her down. This is life and there are decisions to be made that are the best for the situation you are in. It would be sad to the miss the wedding, but the guest list is not up to you.
Now, some etiquette says that breast-feeding newborns should be the exception to the "no children" rule. It's up the hosts if they would like the follow it, however.