Do you know what I would be tempted to do?
I say everyone else throws a screaming fit at the same time. Start crying, screaming, stomping your feet, the whole nine yards. I bet the kid stops real quick because they won't know what hit them. (Ok, maybe in theory. Never tried this myself). But, maybe if the rest of the family members do this, Aunt and Uncle might finally get a clue of their child's temper tantrums and the reactions it does to other people.
If they are in someone else's home, than the owner of that home has every right to tell, not ask, the aunt or uncle to take their kid in another room or outisde until their child's tantrum is over because the family is done dealing with them. Time for them to start dealing with them.