Inviting My Mom On His Family Vacation

My family has a history of somewhat exotic family vacations, but now that my siblings and I are near 30's, we haven't taken one in a long time (>10 yrs). The reason for this vacation starts with the death of my grandmother, who was very close to my parents, and who sometimes joined us on vacation. She left my parents money, and they decided to use it to take their children on what is likely to be our last family vacation -- just like old times, except that they also invited our partners along.

This is a special trip for me and my family, and I just feel like it would be very awkward to have my fiancee's mother along. We don't know each other well (we've met twice), and I feel like I'd have to act a bit differently around her, rather than just relaxing and enjoying myself. I also think that my siblings would be uncomfortable, so that it might affect their trip too. Finally, given that my fiancee herself was invited to join us, I don't think that it's her place to invite her mother.

I wish she could simply say: I disagree with your rationale, but I respect your feelings; and given that it's your family's vacation, I can let it go. Otherwise, a dark cloud could follow us around the sunny Caribbean.