What can honestly be said to people that ignore COVID-19 restrictions?

I have these relatives that are educated, smart professionals and follow the news very closely especially when it comes to the pandemic. They know the risks. They know the dangers. They know their own potential for dying of this virus based on their comorbidities which are Type-1 diabetes, cancer survivor, and several other autoimmune issues. They have all had multiple negative COVID tests in the last year. They have one teenage child. My wife and I are the only relatives capable of raising their child in the event they both die. We are named as the child's guardians in their wills.

Okay, here is the issue... They are on their third vacation since the pandemic began. They've traveled to several states during these vacations, visited other relatives, and spent a lot of time in large metropolitan areas. They go to malls, beaches, water parks, and go out to eat constantly. They believe that they are acting in a safe and responsible manner and that we are too cautious.

My greatest fear, as my wife and I are about to retire, is that we will spend the first decade of our retirement raising their orphaned child. And there is nothing I can say to them that they will hear. It is useless to reason with them and I don't want to threaten them with refusing to be guardians because we are literally the only option they have and it isn't their child's fault that they are behaving this way. Heck, the kid is often used as the excuse for the vacation trips.

What would you say to these relative if they were yours? What can be said that they would even listen to? They already think that they are smarter than everyone else. I just hate the trajectory this could be headed towards.