Sister in law from hell

How do you deal with a sis in law that thinks she is entitled and when things don't go her way play the victim? Does anyone else know people like this... they just want and want and need and need. There is always some great conflict in their life because some one did this or that. My mil is a very nice and fair lady. She is always thinking of everyone else and helping where needed. She is not a girly girl or into showers and wedding planning.. which is a ok w me, but my sister in law always takes the weirdest stuff to heart and thinks my mil (also her mil) is out to get her. I'm going to snap one of these days! How do you tell some one they are selfish and ridiculous!!! Not to mention she thinks that every one has an issue. For example, her husband has a low sex drive so it must be mommy issues... or he is always tired so it must be depression and he needs to see a shrink... it s like NO he works 60+ hours a week so you don't haved to... he is just tired!