Hello Mozart, it's nice to meet you. I don't really have any advice except to try to have a conversation that brings out your sisters logical thinking and maybe she will see the problem. Sometimes just having some help reasoning things out helps this kind of thinking.
Welcome to psych central, if you have any questions feel free to private message any community liaison or moderator, here is a link to a list of forum leaders.
http://forums.psychcentral.com/showgroups.php
__________________
Friendship is born at that moment when one person says to another: "What! You, too? Thought I was the only one." C.S. Lewis
visit my blog at
http://gimmeice.psychcentral.net