Sister in-law causing strife, 42m, 37f, 31f.

I have been with my wife nearly 20 years, and she has a younger sister. This younger sister is 6 years younger, and for the life of me, seems incapable of making good life choices. Anyway, my wife and I have been planning a vacation to Europe for years, its been a long time coming and there were some delays (covid, work, lack of money and vacation time etc). Anyway, we are now in the final stages of planning and are booking flights, rooms etc.

Enter the sister in-law. She gifts a travel giftcard to my wife, several hundred dollars. Didnt think too much of it at the time. But now, she is assuming she is coming with us, and making plans to do so. I have told my wife I have been planning this vacation for US (as in the 2 of us) and she had been avoiding bringing that up with her sister. My wife also does not want me telling her sister no. So here we are, closing in on the date of departure, and I am only getting vague answers as to whether the sister is coming, which in my 20 years experience being with her, means there is a 200% chance her sister will in fact be tagging along. And by previous experience, I mean her sister has done this many times before, just invited herself along, or even getting her own plane tickets/hotel etc, and then expecting to spend the duration of the vacation with us. During which she will quite literally argue about what we are planning to do, insist we do what she wants to do, and if we ignore or overrule her, she will then go off on her own…and generally within an hour, maybe 2, blow up my wifes phone because she got into some kind of trouble or situation and she needs us to come get her.

I have decided I am going to die on this hill. She isnt coming with us. My wife finally talked with her sister, and there were tears, accusations, manipulations, anger, resentment etc. And even after all that, her sister said “she is going to think about it and let us know”. In my mind there isnt anything to think about, she doesnt get to determine HOW I spend my vacation and money or WHO I spend it with. Her sisters emotional and manipulative responses are likely why she was avoiding talking to her about it. I am done having my own vacations side-tracked by her sister. I told them both, if they want to go to Europe, then they can go, and I will make my own plans. If her sister shows up at the airport, on the plane, in Europe, then I will be leaving to have my own adventures elsewhere.

I am expecting an emotionally manipulative email, text or phone call at any time from the sister because of this, who seems either incapable or unwilling to consider anyone elses feelings on this. My wife is not helpful in this matter, as she cannot or will not say NO to her family.

So, now my wife is low key guilting me for making her sister cry while also appearing to be somewhat elated that I get to be the “bad guy” instead of her. So now I get to look forward to her sister being salty AF for the forseeable future. And personally I dont really care, but this will, for a fact, stress out my wife, and that I do care about.

How would any of you handle this situation?