Planning when you have a chronically ill child

Awhile back my sister asked us if next yr we wanted to go on a trip with them, I immediately said yes, not thinking ahead. then it came up a few questions about the trip and it made me think that I don't have the answers , i dunno how my child's illness will be when the time actually comes to go on a trip a yr in . What happens if we cannot go ? what happens if we plan it all out and in the end we cannot due to health issues go and everyone is relying on us to pay our part of the trip and it is now onthem to make up teh money for us not going and contributing to this trip? How do other families of chronically ill children/people plan a trip with friends and family and feel ok about it if the time came and you coudl not go ? I tried to explain this and in the end was not understood by my family. I am confused and not sure anymore if in our life we can honestly say we can do something , to be open and honest about it I woudl feel horrible if we said we coudl go and then couldn't and left our family having to make up for our part of the trip. How do I explain how I feel to the ones who invited us and am I wrong in deciding that we cannot plan things like this in the event we cannot go ?