Slim and Piglet

Why do they fight all the time?? There is nine years between my daughters. My oldest is now almost 16 while my little one is going to be 7 very soon. There is a huge gap between them because I was too young when my oldest was born. However, when I met my husband and we decided to get married and ended up pregnant I thought the age difference would be a benefit. I am fourteen years older than my sister and I always adored and loved being with her. It is not the same for mine. These girls are like wild animals in the jungle, starving, and the only thing to eat is the other one. They just cannot get along for any amount of time. They both believe they are in control of the other and need to be each other's parent, personal police officer and tattle tale. They live to get each other in trouble. I would think my older would be above all of the bickering, but she's not. She's actually an equal instigator. She thinks she's the boss and has to control the little one. She is jealous because she has more chores, and we expect more out of her. She doesn't think it's fair that the little one doesn't have the same rules. She thinks they should be equals. Hello??? And the little on thinks that she can do what she wants when she wants and control the older because she's the baby and everyone loves her!! She believes the world revolve around her. And, God forbid, the older step out of line about anything then she is in trouble because Piglet will run to her daddy, eager to tell on Slim. They're horrible. They can't hardly stand to be in the same room as each other. They bicker and snap at each other over everything. They actually look forward to finding reasons to be snotty to each other or get the other one in trouble. They go out of there way to do it. I just don't understand it. I can't figure out why Slim, who is almost an adult, insists on acting like a child. And I can't figure out why Piglet hasn't figured out that her attitude only gets her in trouble. I honestly believe she thinks it's worth it. No matter how I look at it, they drive me crazy. They are both so selfish and inconsiderate. I don't think either one cares how much it hurts me to have the two people I love most hurting each other every single day. It's has to just be about them all the time. Going crazy, Sissy