Crab Weasley and Goyle Weasley, also known as Crabbe and Goyke, are friends of Ebony's who randomly show up in several scenes. They are likely based on Crabbe and Goyle, Draco's friends, from the books. In My Immortal, Crab and Goyle are members of the Weasley family. Darkness, Diabolo, Crab and Goyle all have the same father. Their father was a vampire who raped them, so they all became gothic and converted to "Stanism."
It appears that Tara Gilesbie may have mixed up Crabbe and Goyle with Fred and George Weasley from the books. In Chapter 17, Crab and Goyle set up a fireworks display from "Wesley's Whizard Wises" - this would be behaviour much more characteristic of Fred and George than of Draco's friends.
Also in Chapter 17, both appear to enjoy crossdressing, as it was mentioned they wore the same clothing their sister Jenny Darkness was wearing, who at the time wore a black dress.