Why doesn't it seem like the enemy is always right about things?

My sister accused me of stealing her necklace and she KNOWS that I wouldn't do anything like that. My enemy knows I didn't do it while I told her during our project we were doing. Then in the next day. My sister and I had a huge argument and I asked my enemy to prove that I didn't do it but she said she shouldnt have to come between family and trust. She harshly said, "Look dont drag me into this. This is family problems. I should not have to be between with a father and his homely, pitiful daugbter." I got angry and got after her and my best friend too. My mom stopped us told us to calm down and said our enemy is correct about what she said and said as it is, I agree. I'm just pissed off that my sister would think I would steal from her. Geez