Was I wrong? Should I apologize?

My sister and I just had a big fight. My sister had these really nice pink pumas and I kind of stole them at the beginning of the school year. They were filthy, so I cleaned them really well and changed the laces and took really good care of them. I kept them at school so she wouldn't see that I had her shoes, and whenever she asked about them, I would tell her I didn't know where they were. Pretty soon we started having PE instead of health and I kept the Pumas in my gym bag in the car, where I knew she would never look. Today we were cleaning out the car and she found the Pumas! She got really mad and tried to fight me, but my mom came in. She then proceeded to call me a and I got really mad and told her that SHE was the one who was a because recently she's been dressing really slutty. My mom told me I should apologize, and naturally after that episode I REFUSED. My sister began to cry and I was like jeez, they were a freaking pair of shoes. Anyway, I feel kind of bad about it now because she was crying. What should I do? I mean she's always been stealing my clothes and things and nobody ever says anything about that. I'm still mad about the whole thing but I want to do the right thing. How can I set things right?