Okay kiddo, let me explain to you what an eighth sister is. Do you know what a family tree is? It's like a big chart that shows how different people in our family are related to each other. In Star Wars, there is a family called the Skywalker family. They have a dad named Anakin Skywalker who becomes a bad guy called Darth Vader.
An eighth sister is a character from a Star Wars book called "The Clone Wars: Stories of Light and Dark". She is not part of the Skywalker family, but she is related to another bad guy named Darth Maul. The eighth sister's real name is Mas Amedda, and she is part of a group called the Inquisitorius. They are like the secret police force for the bad guys in the Star Wars universe.
So to sum it up, the eighth sister is a character who is related to a bad guy named Darth Maul and works for the other bad guys as part of a secret police force called the Inquisitorius.