[SFSkol]

It appears here the key is to stop the email communications. As long as there has been no other method of contact, this might work.

Call your sister and tell her that you have been receiving dialy email messages from her that your mail virus scanner has been catching as containing a serious virus. Have other family members also call her with the same issue. This virus will attach itself to every message she sends out forever, Your sister should then start a new email account, Gmail is fine. Say you believe it is completely unrelated to her scammer. She can transfer her contact addresses. Now the scammer will be communicating with a different address than the original. Should scare them off since they might believe the authorities might be now monitoring it. (or better yet even start a new account without transferring the contacts.)

This has worked twice for me with customers with a cash mail scam issue.

Good luck!

Also: Email providers have a method of blocking specific address from going into the inbox. If you can get on her machines that might also work.