Bit of background DSis lives in Australia and has been living overseas with her family for about 30 years. We keep in touch but are not really that close. We have not met in person for over 6 years and only every 3-4 years before that.
My DD is getting married in September. It is a fairly small wedding, 60 for the ceremony and wedding breakfast with another 40 [mainly friends] coming for an evening party. The decision was made months ago not to invite any cousins from either side. DSis and her DH are invited and have accepted although she made it clear that she was very disappointed that her two DCs were not included. They are both late 20's. My nephew is independent, but niece is still at home with parents mainly due to MH issues. I have heard a rumour that my DSis and BiL are planning to bring the uninvited niece with them presumably in the hope that we will somehow shoehorn her into the arrangements. My DH is fuming to say the least and never had much time for them anyway. He says that this is gate crashing and if they do this we should uninvite them even if they have travelled from Auz. I am not sure how to handle this. Any advice?