Hi Nika Bee, Write your response and then sleep on it.
BTW have you read Steve Hassan's books (i.e., "Combatting Cult Mind Control", "Releasing the Bonds: Empowering People to Think for Themselves", and (his latest book) "Freedom of Mind: Helping Loved Ones Leave Controlling People, Cults and Beliefs"), visited his website www.freedomofmind.com, or watched his FREE videos on his website? Following Steve Hassan's methods may help improve your relationships with your JW family.
Your best response would be to ask simple questions, like why the sudden change of heart and that you love her unconditionally like how Jesus Christ taught Christians to love. If you still have the letter that your sister wrote to you, can you copy it and send it to her. You could even ask her where in the Bible did Jesus Chirst shun or not associate with non-jews.
Best of wishes helping your sister think critically for herself.
Peace be with you and everyone, who you love,
Robert