I do think YABU, you have a choice of getting the train or being driven, but only one will mean your middle child's plans have to be cancelled. I think if there is an option where you can travel with the ill children (whether or not you like it) and your DH can take the other child to their planned trip then you should take it.
Not quite the same but one of my brothers was a schoolboy athlete - he was constantly at training sessions, competitions, trials etc - and me and my siblings always had to work around his training and competition schedules. As a child I really resented that everything had to be dropped for my brother and I never got to take priority, it was always my or my other siblings plans that would get cancelled if there was a clash between our event and my DBro's. Just once I wished that my parents would prioritise me over my brother but it never happened. As an adult I can see how hard it was for them to try and balance us all but as a child I just felt second best and always overlooked. Please don't do that to your middle child.
We can have the MRI on Tuesday but need to travel 7 hours to another country to go back to the same surgeon. My husband does not want to drive us there as this will take the entire weekend and he has ski-ng planned with the middle child.
OP can you clarify this please?
You say the MRI is on Tuesday and the skiing is at the weekend so why would they impact each other? Are you planning to drive the 7hrs over multiple days - Sat/Sun/Mon - which surely would require staying in hotels which are just as much of a covid risk as a 7hr train?
Also, why can't you have the MRI locally, the results sent to the surgeon and then only travel if surgery/physical examination by the surgeon is required?
I know you are abroad so a different system but I am under the care of a consultant in a different NHS Trust as there is no specialist provision in my area, but they are 4hrs away. If I need to have tests my consultant will always try and arrange for me to have them at my local hospital where possible to save unnecessary travel.