From my understanding, MiL is too ill to come out? Perhaps you need someone to sit with her so you are not worried, either a paid carer she has got to know, or a neighbour. The chances of anything happening in the few hours you are out are very remote, but of course the hotel would get you a taxi if you need to get home fast.
But remember you are celebrating your 40th, on that day you would only have had eyes for each other, and whilst it is lovely to have a family meal, either in house or out, perhaps a 'second honeymoon' is even more important.
We had a blessing ceremony in our church which everyone came to (didn't feel we needed to renew our vows as we hadn't broken them!), then tea and cakes in a nearby hall, and then on to a restaurant for dinner, at which the guests were happy to pay for their own meals. The following day we left for a short break away, and the whole thing was lovely.
Have a wonderful celebration, and congratulations when you get there!