AITA for refusing to babysit my sister's kids after she told me I was "too old" to have dreams?

I (28F) have always wanted to be a professional dancer. It's been my passion since I was a little girl, and I've trained hard over the years. I understand that it's a tough field with no guaranteed success, but I've managed to land some gigs here and there. I'm not famous, but I'm proud of what I've accomplished and I continue to work towards my goals.

My sister (34F) has two kids, ages 5 and 7. She is a great mom, and I admire how dedicated she is to her children. That being said, she often asks me to babysit them, especially when she has events to attend or needs some time for herself. I usually don't mind helping out, but I do have my own commitments and sometimes I just can't.

Recently, she asked me to babysit for a whole weekend while she goes to a music festival. I told her that I couldn't because I have an important audition coming up and I need to focus on that. She got really upset and accused me of prioritizing my "stupid dreams" over my family. That's when she said that I was too old to have dreams and that I should just give up on dancing and help her instead.

That hurt. A lot. I tried to explain to her that I am not "too old" and that people can achieve their dreams at any age, but she wasn't having it. She called me selfish for not wanting to babysit and put her kids in my place, saying that I should be ashamed of myself. Now, I love my niece and nephew, but I also think that they should learn to respect other people's boundaries.

Now, she's telling our parents that I'm a bad aunt and I might ruin their relationship with her kids if I keep refusing to babysit. My parents are somewhat supportive of my dancing career, but I can tell that they think I should help my sister more.

AITA for standing up for my dreams and not wanting to babysit?