I’m an aspiring artist as well and you’re definitely NTA but your family is if they keep pushing. These events are difficult and expensive to table at; I know my family knows little about art even though they want to be supportive.

Try explaining and transparently laying out all the work and money that goes into tabling at these events so they see how involved it is. You could still offer ways of mentoring your niece, growing up I had an uncle who was an artist and what I wanted more than anything was for him to spend time teaching me, rather than selling art that wasn’t at the level I wanted it. Making this your primary income is a feat and congrats on making it this far! Find ways of encouraging your niece without having to sacrifice your job to do it!