I said to my wife: "If you talk to your sister, you are divorced." Does divorce take effect?

If the husband says to his wife: "If you speak to your sister, I will divorce you," this is a threat and a future warning. Divorce does not occur unless he intended to effect it when she spoke to her sister. However, if he only intended to threaten or prevent, then divorce does not occur, and he may be required to offer an expiation for breaking an oath.