S.W.
I am an only child and have three sons, so it's been a new thing for me to learn about sibling relationships. I highly, highly recommend the book "Siblings Without Rivalry." It helped me understand where some tension was coming from and how to address problems without setting up further rivalry. It's an important book to read, but it's a quick and easy read you could probably find at the library. One thing I learned is to pay attention only to the victim in a conflict, so there is no motive of getting someone else in trouble and no attention or control for picking on a sibling. Also, I learned that punishing a child because of meanness to a sibling can fuel resentment and rivalry because it's perceived as the sibling's fault the child is punished. . . the punished child just plots revenge, which wasn't the intention.
The book "How to Talk So Kids will Listen and Listen so Kids will Talk" by the same authors also addresses sibling conflicts and is the book that saved my sanity. I really, really recommend it. Hang in there!