Child-free weddings can work, and destination weddings can work, but a child-free destination wedding when the groom's three siblings ALL have young children, was a pretty terrible idea. It smacks of your fiancee trying to exclude your siblings right from the start, as it's picking a wedding plan that is asymmetrically burdensome only on your siblings, and not on hers, since her siblings don't have young children.
Claiming that you accommodated your siblings by offering to have a family member babysit the kids during the wedding is a bit of a red herring here. You were asking your siblings to pay multiple plane fares, for themselves, a spouse, AND their children, and presumably hotel fees, in addition to all the usual expenses of attending a family wedding, and to go through enormous hassle (traveling by plane with young kids is very burdensome) and then have their kids babysat at the wedding which would also be stressful, and the whole time there's this attitude that their kids are not welcome at the wedding. Ugh. And that's beside the entire aspect that, who exactly was going to babysit the kids? You were offering to fly out yet another in-law to babysit? So in addition to all your siblings, their spouses, their children, having to make this big trip just to be at your amazing wedding where part of their families were unwanted, they have to beg another relative to also inconvenience themselves to make an international trip to babysit the kids that were unwanted at your wedding? This relative would surely be more than delighted to undergo the preparation, packing, having a cr@p meal, and no fun, and babysitting three kids under 2 (!) so that you could have the child-free wedding of your dreams. Of course.
Or, well the alternative could be that any of your siblings who didn't feel up to this massive trek, wouldn't have to come to your wedding. Which, I hope you realise, might have ended up as your wedding celebration having all HER siblings present, none of whom have kids, but none of YOUR siblings.
Do you not see the way this wedding plan favors her family over yours? And you're surprised your family weren't over the moon with delight at this plan?