My DS was 18 months when my DD was born. DS was meant to go to a neighbour's house but when it came time I couldn't leave him, I wanted my boy with me, he had never left before. I was so proud of DS, during the birth he just watched a dvd and played the whole time. He occasionally came over and checked me then went back playing. As soon as DD was born DS came over to meet his little sister, he gave her lots of smiles. He adores his sister and is very protective of her, I think him being at the birth helped him take to her so quickly. We had someone else organised to care for DS in the labour ward/hospital if he came with us but when it came time it was 3am and they wouldn't get out of bed. They said the car had a flat battery but I don't believe that for a second.
Whatever you do it may be important to have a back up in case one falls through. I hope you can get it worked out quickly.