This is absolutely untrue. The sister you refer to (Anne's sister Elisabeth) had a very severe heart condition which killed her when she was still a young woman. She was perfectly fine, mentally, and this story was made up out of whole cloth. Anne also had a brother who was a diagnosed paranoid schizoprhenic (though that diagnosis was overused during the early years of psychiatry). Why does no one point fingers at HIM? (He didn't do it, either.)
I have studied the Lindbergh case for years and am convinced beyond any doubt at all that Richard Hauptman was, indeed, the kidnapper/murderer of the child.