Is the wife entitled to prevent her husband from marrying another, and what is the ruling of Islamic law on her threatening him and his future wife?

If a man desires to marry and is capable of upholding justice, then there is no objection to polygyny for him, and the permission or notification of the first wife is not a condition. We advise you to perform Istikhara; for if there is good for you in marrying him, Allah will facilitate it, otherwise He will turn it away from you. If the marriage is facilitated, then that is good and excellent. Otherwise, do not pursue this man, but rather seek someone else and sever all ties with him to avoid temptation.