The very same thing happened to me back in July, but my sister never would have joined a discussion group like this. However, over the months, she has come around a teeny bit. She has sent me a letter, called me, and accepted a call from me. It's not nearly what I want, but I'll take what I can get. She's the most important person on this earth, to me.
Give her some time. If she had enough doubts to join JWD, she still has them.