You could substitute many words to describe the family (artistic, sports-minded, gymnasts, computer programmers, farmers, medical professionals, mechanics, educators) and also the child whose interests are either a surprise or a shock or a source of confusion, and the scenario could be the same. It's not just "jocks" and "art". For example, everyone in the family is in the healthcare field and then along comes little Annabelle who hates the sight of blood and who knows from an early age that she is going to grow up to be a tap dancer. Or, everyone loves cooking and food and the family runs a restaurant and then there's Zachary who can't even (and doesn't want to) make toast but who is captain of the varsity swim team and spends every spare hour at the pool, training.
Sometimes it can be a pleasant surprise (Who know we'd have an athlete in our dorky family that stumbles over invisible stuff and can't even throw a crumpled piece of paper into the wastebasket with fewer than 12 tries?). Sometimes it's a shock. (But we've ALWAYS sent our children to the University of _____ and we have season tickets to all their games and our house is decorated in their team colors! And now our son wants to go to _____ College? It's unheard of! What will we tell the Alumni Committee?). And sometimes it's downright scary or confusing. (Isn't doing that dangerous? Can you make any money doing that or get an actual job? You want to study WHAT? What even is that? You want to be a WHO? What do they do? You'll have to study WHERE?).
So maybe you can just say something about how delightful it can be to have a child who knows his or her strengths, or perhaps express your encouragement by acknowledging how it can be a bit scary when your child is so talented but it's something you are unfamiliar with. If they use words like "weird" or "kooky", just say "aren't we all?" And ask the child to send you drawings, or even commission a drawing, by saying you'd love a drawing of your cat or dog to put on your desk. The child, hopefully, will perceive your support and feel encouraged. I wouldn't use the words "fear" or anything like that with the family. They already sense that. Instead, put different words or ideas into their minds that they can think about. Focus on the positive and only go there. And I wouldn't limit it to art. The art isn't the issue, it seems. It's the idea that their kid is going to go into a completely different direction than the one they've always taken, or what they expected. It can be unsettling, and perhaps they're worried that they won't be able to talk with her about her career, or even understand it. The kid knows that, or will. What will matter is that they respect and appreciate her.
My husband is retired career military. I have a degree in education and have done a lot of tutoring in English as a Second Language at various levels (mostly volunteering due to our daughter's medical needs, so it wasn't really a job, but it's where my interest and skills are). And our son? He's an audio engineer and soundboard systems technician (a WHAT?, we said). We still have to look up things that he talks about, just to understand the basics, or to be able to simply talk with him. It's like a foreign language. He didn't have music lessons, and we know nobody in a music or audio career. But he knew without a doubt that he wanted to be a sound technician and found a college and graduated with a near perfect GPA and now works full time in his chosen field. My husband always assumed he'd join the military. I pictured several careers that he'd be good at. The music field and audio engineering never entered my mind. But thankfully we realize that this is who he is, and it's what he loves, and he's good at it. I hope the same thing will happen to your young family member.