We had a similar situation. My mom and her (now ex) boyfriend of 10 years separated a few months before I sent out invitations. I spoke with both of them before sending invitations that we are choosing to have a small wedding. Even after our conversations, my mom's ex invited his sister and her husband, who were not invited and not part our our guest list. He did not tell me this. He told my mom and both of my sisters. I decided to call him on my own and explain that we only have seats for the people who were on the guest list. He said that they were only coming because they wanted to take a vacation. I really do not know why they would choose to come to a place where they were not invited. But, like you said, we cannot control where people decide to vacation. I do not want to make the trip awkward and leave anyone out, but we simply do not have the space for them. Our trip is in October, so hopefully it will go okay. Good luck with your wedding!