I would go, and see how the event goes. With other relatives, and maybe residents at a less advanced stage, it may well be a better occasion than you are thinking. I always find with several people to make it as lively an event as possible, it gives enough good atmosphere and distraction from the sadness behind the illness and even less aware people react to a happy lively atmosphere which is do difficult to achieve on your own. You can always make an early exit if it proves not to be like that.
