Who is right?

My two sisters never decide on anything and never get along. They always bicker. The older one always tries to be nice to the younger one. Yet, the younger one just responses in mean ways. She tries to get her into trouble, gives her a hard time, says she hates her. The older one scan't handle sometimes and freaks on the younger one, saying she hates her two. The older one does helps the younger one however she can. Although today's fight could have been pointless but I don't know. My mother brought home doughnuts. My little sister said if she could have half of my other sister's doughnut, the older one said no. However my mom said she could. The older one got annoyed, how the younger one gets everything, she always has to have everything. Take into consideration they are only 13 and 7. I really don't know what to think. When my mom left, the olger one said she doesn't wan't it anymore and will just throw it into the trash, then the little one says that's wasting, then starts crying,