#14 - Feb. 20, 2011, 10:21 p.m.
The others have already covered the basics - definitely a phishing attempt.
While the below article is normally reserved for suspicious e-mails, it may also help you identify suspicious whispers as well - many of the mannerisms, such as threatening to close your account unless you do XYZ is usually the same.
How to Identify Fake or Phishing Emailshttp://us.blizzard.com/support/article/25133You may also want to show your sister this article, as it describes and shows what true conversation chat from a Game Master looks like. >^.^<
How to Contact In-Game Supporthttp://us.blizzard.com/support/article/19775