Things that irritate me

I love my children. Really I do. But I cannot stand it when they fight. It starts from the moment they walk into the door from school, and lasts sometimes past bedtime. It mainly comes from both of my sons. They both love to tease and torment their sisters. Could someone please explain to me what is so fun about making someone else cry???? And lately it seems like all they want to do is fight. AARRRRGGGHHH!!!