I truly believed that they hated each other, as you do now. So I can't explain it. It's just a fact of childhood. There are people who study this phenomenon in books and might come up with some psychological explanation but in the end it's just the same. There is no solution.
Time is all it takes.
Keeping them apart works when they are babies but as they grow up they choose to be with each other, and they choose to make life miserable for each other. The older girl is doing her part in making the younger one furious. It takes two, and each is playing her chosen role. Even if they realized how miserable it makes you, they wouldn't stop. It's possible that your misery is part of their need.