Does a father's request for his children to visit his second wife, after he agreed to the mother's condition not to force them to do so, constitute a breach of the covenant?

You must honor both your parents equally. Your mother has no right to prevent you from visiting your stepmother due to her jealousy, and if you obey her in that, you would be obeying her in an act of disobedience to Allah. Furthermore, the condition she imposed on your father is invalid because it leads to corruption. The son of your stepmother is your brother, and maintaining ties with him is obligatory. Abandoning your father's house is an act of disobedience. You must make your mother understand that your father's marriage is permissible in Islamic law.