I don't want to tattle on my sister, but it might be necessary...?

As much as you don't want to tell, maybe it's a good idea to casually mention your concern to your dad. Tell him, i don't want any trouble, you gotta talk to her in a way which won't expose the informant and make it general, so you catch her in her tracks. He'll probably know what to do, and you've done your part. The key is to be subtle