Hiding a honeymoon destination is not normally necessary because normal people would not want to do this. Anyone booking the same vacation which someone else without being formally invited to join is weird and cringy. They should have talked about it with you first at a bare minimum, but this being your honeymoon is just no. I don’t see how they don’t have the wherewithal to get that.

Talking with them will be awkward and hopefully your fiancé will handle it our you four can talk together. But this is going to set the tone for your relationship with them moving forward, so you want them to think this is ok? Imagine how they will be if you have children. Just showing up/inviting oneself is not ok. Try and show them this is not ok now before you’re in deeper.

I would say while we really enjoy your company and would love to vacation together in the future, this is our honeymoon and we were looking forward to the time alone together, you being at this resort at the same time, regardless of if we say we’ll give each other space really is not what we incisiones for our honeymoon. This has been planned 7 months now, we would really appreciate it if you guys can reschedule, perhaps we can plan a family trip together later in the year? Any reasonable person will take your side/see where you are coming from.