This is not a situation of you making a plan and then canceling if something better comes up. This is a situation were both parties don’t have solid plans and they both release their schedule at the same time. Yeah it happens.
People just don’t know how to say no. They also don’t know how to say yes. Heck, people don’t even say maybe anymore, they simple don’t respond. If you have two conflicting plans on your calendar and both are very important then someone will be disappointed. If you are required to be at both places who do you let down? What is the best method? Do you tell them early and hope they understand or do you wait until the day of the event?
If you inform early you run the risk of them slathering on a think layer of guilt of how their event is not important to you. If that happens here’s what you do. A few days before the event, tell them that someone in your family died. Ask them again if you are still required to be there or can you go bury your family member. Either way you probably want to break off that relationship.