@Alicia870
Very selfish and thoughtless of your brother. I would definitely decline, and tell him why too. I hate child-free weddings, and IME, it's the most miserable, uptight people who have them! It's also very inconsiderate as there are going to be some people who want to come, but can't, as they can't get childcare for the duration. Them expecting you to dump the kid on the grandparents for FIVE DAYS is a joke.
Weirdly, I know a couple who are going to a wedding in Croatia in 3 months time, and THAT is child-free too. I wonder if it's the same one! 
The woman in the couple (a colleague of mine) said similar; that the bride and groom have invited siblings and cousins who have kids, (but not the kids,) and they have said they're not going! It seems there's more friends and work colleagues than family at the wedding.
35-ish people will be there, and about 25 of them are friends and colleagues. Only about 10 of the couples 40 joint family members are going, because they either can't afford it, or they have kids and can't go to the wedding! So the majority of the wedding guest list will be people they will probably have nothing to do with in 10 years time, and very little family. Seems a high price to pay, to have a destination wedding, that excludes children. 