Pb&j, you really are not alone in this. I am going on a Disney cruise next week with family and am going through much of the same process as you. I do not use a Foley, but I am double incontinent. Although I am double incontinent, my wife and I are very experienced cruisers. However, as I now also have increasing mobility limitations I am not looking forward to this cruise except for seeing my granddaughters being excited and having a great time.
As I have told all of my adult relatives that I am incontinent and wear diapers, that is no longer a stress for me. I strongly recommend that you tell your brother and sister the facts of how difficult it is for you. There is no need to tell them any details. You probably will be surprised at how supportive they are and how they respect your privacy.
Re the logistics, I feel your pain. I will fill a separate suitcase with diapers, absorbent pads, rash cream, etc. I do not have to worry about TSA and a Foley and supplies, but in my backpack I will have not only some diapers, but also a complete red rubber bag enema set with hose and nozzle. I have to carry this in my backpack as daily enemas are how I manage my fecal incontinence and I cannot risk losing a checked bag with the enema set. Imagine my embarrassment if TSA suddenly pulls that out of my backpack!
Remember what you already know - people really don't care about your incontinence. Take a deep breath and enjoy your Disney vacation.
--John