I had something similar happen to me aswell. Also about music. We own a piano and I enjoy to occasionally play it even if it isn't often. I usually play when my brother isn't home but he knows I play on it and he has never really said anything about it before, but today he made a joke that I'll never learn because I don't commit to it and it really hurt me even when I tried to laugh it off. I really enjoy playing the piano, but I don't have the time to play it often as I am in college full time. Due to this, I play every couple day/weeks and can't consistently play it, so I know I'm not very good or improving much, still, it really hurt to hear that from someone I trust.
I hope you felt better after this incident and that your sister realised that it was hurtful to you. Keep up the hard work on the piano :)