Sorry but you WILL get upset - and its because you care! Expect the worst and then hopefully things won't appear so bad on the day. Thats a bit of a downer isn't it but its another stage you have to face - keep reminding yourself why its happening.
Have you had the experience of leaving a little one at nursery school for the first time? I remember working myself up for days beforehand, rehearsing it in my head over and over and then hitting the auto-pilot button when we set off in the car. All of the previous weeks efforts to plant positive thoughts into my little boys head about what the experience was going to be like probably got left at home. He wandered in there clutching my hand tightly with big eyes searching the room for some familiarity and having been brave enough to let go of me for a moment to meet the supervisor I made a run for it. I felt so sick as I glanced at the window to see the lady holding him up as he held his arms out towards me with tears streaming down his face but I knew I had to keep going so I smiled as brightly as I could, waved and blew a kiss and gave him the thumbs up sign - got in the car, drove away and proceeded to sob all the way to work. When I called back to collect him later I could have been forgiven for not believing the supervisor who insisted that as soon as I had gone from sight he had been fine. Fortunately a friend of mine was also helping out that morning and totally unaware of the tears we had both shed, innocently reported back to me just what a wonderful time he had had and how well he had adjusted.
Now I'm not suggesting that Mum-in-Law is going to have a wonderful time but what you see when you leave and what actually happens when you've left may well not relate.
When Dad had to go into respite we only had a couple of days to prepare ourselves mentally. We told him (honestly) that he had to spend a few days at the "hospital" while they sorted out his tablets. Mum visited every day and I almost every day. The days turned into weeks but at each visit we made positive sounds about getting better - little did we realise that his heart would suddenly fail him and he would never come home with us. However the experience was probably much harder for us than it was for him. The drive there and back got longer each day and sometimes he would sleep through our visit. Each time we would have to hit that auto-pilot switch when it was time to go repeating the same words as we parted, desperately wishing we could have been taking him with us but knowing he was in the right place and being cared for by some wonderful people.
I haven't given you much to help prepare mum-in-law have I? Guess thats because its you who will need the strength. I would continue on the "holiday" story, planting little positive thoughts, perhaps even bending the truth a little to say that maybe they'll have a vacancy for you as well and you'll get to stay for a holiday!
You can do it - you have to - for everyones sake. You WILL find the strength - you will have developed masses of it just to get you this far.
Good luck
Kriss