Sounds like you have been letting her get away with it...you need to lay down the law.
First key to any conversation that goes on that may possibly be defensive is to get the person in question out in public. The mall, or somewhere there's a lot of people - then they have a tendency to not be loud or argue back as much! If you can afford it, take her out to a nice restaurant!<br />
Tell her that life isn't all about her! Tell her that if she can just fake it, even for a couple of months, just for her dying parent's sake she could really make mom and dad happy! Typically, when people fake something long enough, they actually start to feel and believe it! who knows? It could turn out to be a really good situation.<br />
But it sounds like she needs a swift kick in the ***, and a good reality check! You're probably the best one to give it to her... Even if she hates your guts after mommy goes bye bye, then so be it! Until then, she needs to push her pride aside and focus on how she's making everybody feel!<br />
Sorry, both my sister's are the same way - we've had on and off friendships since we were born! It takes this - trust me!