Is lying to a girl who was assaulted in her childhood by claiming that what happened to the questioner's sister happened to her, and attributing the assault to the cousin, considered a form of slander (qadhf)? And what should she do for her repentance to be accepted, knowing that she abandoned this girl after she started to return to her life?

We ask Allah to grant your sister and your friend patience and to compensate them with good. Lying is not permissible as a means of inviting to Allah; it is forbidden and must be avoided, and you must repent from it. You could have narrated the story in the following way: "I know a close female relative of mine who was raped," without specifying the rapist. Your words about your cousin were in the form of informing, not direct accusation, and informing about someone's adultery is backbiting, not a direct accusation. Repent to Allah for fabricating this story and regret it.