Summarize this article in one sentence.
The dynamics of a sister-in-law (S-I-L) are complex at the best of times but no more so if she has spent much of her life causing her immediate family to run to her beck and call. The drama queen thrives off drama and having everyone pay attention to her as a result. Sit back at your next family occasion and simply watch. Notice how she interacts with her family members, and how they in turn react back. If you witness a lot of step-toeing around her and acquiescing to her, she is clearly used to getting her own way. Consider what happens when she raises a drama-filled topic. Do other family members rush to agree with her about how "outrageous" the price of child care/electricity/shampoo/dog grooming/car maintenance/etc. is? Do they confirm her quibbles as quickly as possible, thereby engendering even more complaints? This shows that they enable her complaint-filled view of the world and sadly, have long been used to pandering to it. You can't change them but you can set a new role model by not complaining yourself. Notice what happens when you disagree with her. Does she pout, throw an adult-style tantrum or try and put you down? While it's important to stand your ground on things that matter to you, if she does react childishly, you'll need to learn how to manage this carefully. Learn not so much to disagree as to fail to agreethere is a fine line but it's about acknowledging her underlying need (notice me, care about me, help me, etc.) without buying into her view of the world. Your S-I-L can let off steam, vent away and curse all she wants but there is no need to join the negativity. Avoid taking any of what is said personallythe crazier the reactions and actions, the more your S-I-L is grasping at straws to try and provoke you and restore the limelight back onto her. Let her have the limelight in her own home but don't hang around to be vented upon. If it gets really bad, simply announce that you will come back when she is feeling calmer and leave. Equally, if it's happening in your own home, tell it's time to leave. (You can even make up a fake appointment or an early bedtime if you really need a polite excuse.)

Summary:
Realize that you may be dealing with a drama queen. Do not get involved with the drama.