This is not a good idea, you are not being unreasonable.
I think your dh and in laws are being unrealistic. MIL is seriously ill and considering taking a holiday which, from your description, is totally unrealistic given the nature of her health concerns.
I wouldn't consider this at all, it would not be a holiday.
I would consider a few days away in the UK, where you speak the language and if something unfortunate was to happen you are close to home. A few days away could be relaxing and fun.
Being couped up in car, likely with a bored and noisy child (been there!) is horrendous for everyone, and mostly stressful for you because it will be your job to peacify.
I would compromise and offer to go away for a few days locally with firm agreement on where, expectations and wish list for all concerned so that everyone is singing from the same hymn sheet. Otherwise this is a recipe for disaster.
Sorry for the doom and gloom but my dp for a number of years had a very romantic view that going away en masse with the family would be great craic - where everyone would be 'jolly good eggs'!!(slight exaggeration but ykwim) Experience has now taught him differently!!!
If your dh is still keen on going foreign, then that is the holiday that you and your family unit have on your own. Stay in the uk for anything with the in-laws and stick to your guns othewise this will be something that is routinely re-visited by all X