I'm going to tell my sister her music taste sucks because she made me cry
Why did your sister make you cry?
She told me I was ugly.
It's not nice to make fun of someone else's tastes. You should respect people's opinions.
Fine, I'll never speak to her again.
Your both angry at each other over very small things. Usually the best course of action is to just wait it out. You'll both come to realize that it is not worth fighting over and even forgot what  all the fuss was about anyway.