Telling mom and dad about what xat.com is. How do I do it?

Why are you hesitating about telling your parents what you know about the chat box? If it is misrepresenting itself, they should know, and so should your sister, if she doesn't already. I sort of suspect that she does know about it, and is deceiving your mother. I know you don't want to snitch on your sister, but in the long run, it would be for her protection if the site is a sinister one. You can soften the blow by telling your parents that maybe your sister is unaware of what the chat box really is, therefore giving her the benefit of a doubt, so you wouldn't actually be accusing her.