That was truly inconsiderate of your relative.If you ask me to travel two hours to where you live and do a dinner party for you, please give me more than a days notice before you CANCEL ON ME!!!
I'm full of rage. Everything is packed. Hours of work, planning, list making, shopping, money spent, etc. Gone. For nothing.
He's a relative, so there's not much I can do except silently simmer over here.
If it were me, I would use a text editor to compose an email (not an email program, where it could accidentally get sent before I was ready). Then I would wait until I was calm and not nearly so angry. I would work on it. I would try to explain about the wasted time and money. Then I would set it aside. I would look it over again later and maybe edit it some more. I have learned not to respond too quickly when I get annoyed/angered/upset. This is obviously a relative that you like well enough to do this for.