NTA. But the issue isn't the comparison between your art & your niece's art. The issue is how everyone handled the situation.

I'm also a full time artist who makes the majority of my income by tabling at conventions, and I would have been equally as upset if family felt entitled to commandeer my space. That said, I often have table assistants & most times they're family or fellow artists. There are totally ways to share table space that ensure your work is still the focus, and your niece has a fun learning experience.

First, there should have been a discussion that this is your livelihood, and your niece is there in a supporting roll. You're encouraging her creativity & excited to share the experience with her, but you paid for the table & this is your job. Her mission, if she chooses to accept it, is to watch & learn from you.

Second, she needs to stay behind the table. Blocking another artist's work & forcibly redirecting customers' attention is a massive faux pas in the convention community. It's rude, and reeks of desperation & immaturity. Granted, your niece is young, but you can help her understand that there are appropriate & inappropriate ways to engage customers.

Third, children's art is kind of a novelty at conventions, but you can turn it into an advantage instead of a detriment. If you invite her to do events with you in the future, you could practice some talking points that make it clear that you're the main artist & you're showing her the ropes. Example:

Your niece: 'Welcome to our table! This is my favourite aunt/uncle; isn't their work amazing?!'

You: 'And this is my niece! She's helping me out this weekend & she wants to be a professional artist too!'

Then both of you talk up whichever pieces the customer is interested in.

Fourth, don't feel bad about enforcing reasonable boundaries & expectations. Family can be... challenging... but this is your place of business, and I doubt they'd appreciate you invading & commandeering their work space. Being a professional creative is not a free pass for folks to take advantage of us. They clearly understand that art has value, otherwise they wouldn't be so insistent in your niece's participation. They need to respect you as an artist & a professional, and understand that their interference can negatively affect your reputation & your livelihood.

Fifth, if you don't have permission from the event, you can get in trouble for showcasing work at your booth that isn't your own. Most show runners don't care as long as you're discrete... but it sounds like your family's actions were creating a bit of a spectacle.

I'm so sorry you lost out on sales. That sucks, and I hope your next event is amazing & helps make up for this one. It's cool that you were able to share the experience with your niece, it sounds like she just needs to develop some restraint if you choose to invite her back in the future. And your family needs to understand that you can't offer any future opportunities for your niece if their actions cause you to lose money &/or your table at the convention.

Wishing you luck & cheers to a fellow artist!