There's not enough information here to help nor is there any leverage OP (or really the parents) could use to change their behaviour without threatening their r

There's not enough information here to help nor is there any leverage OP (or really the parents) could use to change their behaviour without threatening their relationship with the kid. If it is an addiction like behaviour the person needs to see the negative impact their addiction has on their life and what they love before they are willing to change. I bet this kid had free access to screens since they were very little. Why doesn't this kid have a job? Why doesn't the kid have a social life or other responsibilities that takes them outside? Are they special needs, physically or mentally? Are they being solely supported by their parents? It sounds harsh, but tell your relative "The horse has long left the barn."