In addition to her video, a well constructed résumé of her dance and violin performances with variations/pieces studied could underscore her mastery and dedication. Perhaps address how the pursuit of the two art forms enhanced both. My daughter did the same.
If your daughter has any interest in choreography and can create something before applying next year, she should consider adding that to her arts supplement. These organizations need leaders and those who can create original content for performances.
Columbia, for example, has a number of ballet organizations on campus. Some do rep and classical variations while another produces new choreography. Many of their students study challenging degrees as do those at Harvard, Princeton, etc. One doesn’t need to dance 20 hours at college to do ballet either. Yale’s ballet club is newer, but the school values the arts highly and I believe is trying to make progress in ballet.
Finally, I think creativity is an important asset for STEM, as it is for almost any field or life in general. Address that directly…they are not mutually exclusive.
Your daughter has great academic rigor, which she has succeeded in while accomplishing many challenging ECs. Admissions for the top schools has become exceedingly difficult, but essays and supplements that thoughtfully reveal her development, intellectual connections, and growth should make her memorable. The rest of her application is solid. Good luck!