I agree with someone else that saying that you wanting to "discuss" might be making it worse. Sure, discuss where you're eating, what you're doing, but wanting to discuss how they are going to handle issues if they come up is, frankly, none of your business (to be blunt).
I've traveled several times with my ex husband. We get along much better now that when we were married (it's much easier to overlook things knowing you can go your seperate ways when you get home). We've also traveled with other people and there have been no issues because we are adults who are doing what's best for our family.
If your sister and brother and law are adults, then trust them to handle their personal business personally.
I disagree, how to handle issues that impact her and her family on their vacation are ABSOLUTELY her business. If her sister and brother in law were adults, they wouldn't blame her for taking a Disney trip away from their kids for just wanting to talk about it. They've already proven they aren't adults.