Am I overreacting being mad at my sister for name calling to parents

I’m 18F and my little sister (15F) always asks me to do or “help” with her homework last minute. I usually help, but yesterday I said no because I had my own stuff to do. She got super mad and told our parents I was “never supportive.

I tried explaining that I have my own deadlines and responsibilities too, and that helping her last minute isn’t fair to me. But my parents seemed to side with her, saying I should be “more understanding” and that she’s stressed. I feel like I’m being treated unfairly for setting boundaries, and now everyone’s acting like I’m the bad sibling.