I would not confront her, the relationship is between she and your sister and she is your sister's "problem". You have stated how well your sister has helped you, I would let her fight her own battles, it sounds like she is quite capable.

As far as your relationship with the professor, I would now see her in a "fuller" light, not just as good, kind, and helpful.

I had a doctor I almost sued as her lack of attention/care in a medical situation where she was charged and accepted that attention/care caused additional pain and suffering and could have killed me if not addressed by another doctor. Unfortunately I had earlier recommended this doctor to my daughter-in-law who hired her and liked her.

The moral is that one gets the opportunity to see a person more completely; presumably your sister now knows the professor has a thing against violinists so she does not have to take the professor's comments so personally as that bias is now known and personally illustrated and you have a grain of salt to evaluate her with when she comments on your work in the future. While it is difficult to operate under and learn from a cruel or prejudiced teacher, the experience can be used to strengthen one's self-esteem and focus one one their personal work, knowing the criticism is not entirely valid and the need to judge one's own work and how happy one is with it is more important than accepting other's judgments of it.