Thank you for the question. When you say that you cannot send or receive messages to/from your sister, what are you referring to? Are you referring to email or another method (ie - AIM, Yahoo Messenger, etc..)? There is no reason why you wouldn't be able to send/receive emails between Hotmail and GMail, so I'm just hoping to understand the problem more clearly at this point. I look forward to your reply. Thanks!
I would suggest sending the mail through a gmail account with an @gmail.com address, or sending directly using the mail.yahoo.com web page. Yahoo does not allow mail to be sent on its behalf (ie - from a web contact form or through gmail). Sending directly should allow the message to be sent. Let me know if you need help with that and I'll be happy to start a remote session with you. Thanks,