It's funny that we live in such a wacky time that we somehow believe that posting words on facebook is considered as revenge. Revenge used to consist of actual repercussions. Just post really sarcastic statements likie " Have I told you haow absolutely awesome my sister in law is, she is so awesome that she did________ to me :)
Trying to get revenge on ANYONE can only end badly. And to try to get revenge on your sister-in-law? Sheer idiocy. It may feel good for the moment, but the repercussions will last much longer than the instant gratification you would get in that instant when you press the "submit" button. Then again, if your goal is to tear your family apart, then damn the torpedos and full speed ahead! Get used to being lonely and distrusted.
If you're trying to "prank" her, you could post funny comments on her page like, "When is due date for the new addition to your family" or "Congrats on winning the lottery" or "Do you need some help moving?"