How do I handle my difficult Sister-in-law?

Well I think you hit the nail on the head when you said families never leave each other - which means you should maybe offer her continuing love and support whenever she's ready to accept it. That doesn't mean you have to give in to her demands, just that you underline the fact that you will always want her in your life no matter what. From your description of her it sounds like you guys aren't that positive about her and she may realise this. Hopefully over time she'll grow out of being selfish. Personally I'd avoid discussing the details with her, send her a new friend request on facebook and wait...