What a pity. This is a story that can be repeated many times over in Jehovah's Witness families.
The core of the problem is honesty. Your sister isn't honest with herself or others. She is in denial. She blames everyone else for her unhappiness. She clings to one thing that appears stable ... the Organization and the promises it promotes. We know this is an illusion.
Maybe this would be a good time for you to be honest with her. Don't be judgemental. Don't try to make a point. Let her know that you are family - and you're here to help her in anyway possible.
She is having a major life crisis. Her 'friends' in the congregation will drop her like a hot potato. She'll need your unconditional love and understanding.
Good luck!
ginger