What is the ruling of Islamic law regarding a man marrying his deceased brother's widow, knowing that he is already married and has children, and that his wife rejects this marriage? And are his emotional conversations with her that reach the point of sexual innuendo permissible before marriage?

What you have done by engaging in phone conversations with your deceased brother's wife, and the relationship developing into an exchange of emotional words, is not permissible in Islamic law due to the corruption and temptation (fitna) it entails for women. Therefore, we advise you to repent first. After that, if you wish to marry her and are capable of fulfilling her rights and maintaining justice between her and your other wife, then there is no sin upon you, as Islamic law has permitted a man to marry more than one woman.