I have to work with my sister?

My sister had a job for a few years until she moved out. About 5 months ago I got a job where she use to work. I was very excited, because my boss was really excited to have me. Sooner than later, both me an my boss realized that my sister an I are nothing alike when It comes to working. She has alot more experience and is quiet and gets the job done. I am young, talkative, and think things through and ask questions before I get the job done. My boss is disappointed in my turnout- but I can't help it. I work my hardest, and never slack off. Recently my sister moved in on a whim. She just showed up on our doorstep. We don't get along very well. I'll just say this. She asked me like 6 times if I would be willing to work with her. I clearly said no, because my boss clearly favored her over me and that's how it is. But today she went behind my back and started working. She has the exact same hours as me. This makes me extremely upset. How do I deal with it?