What is the Sharia ruling on a son's obedience to his father and cutting off ties with his divorced mother, even though honoring the mother is the option favored by Islam, and are the stepmother and siblings sinful if they do not obey the father in shunning the son?

You must be dutiful to both of your parents. It is not permissible to obey one of them in severing ties with the other. You have done well by maintaining ties with your mother and by not obeying your father in severing them. His wife and children are not obligated to obey him in severing ties with you, and there is no sin upon them for speaking with you without his knowledge.

You must maintain ties with your father and be dutiful to him; it is not permissible for you to sever ties with him. Wisdom and gentle persuasion should be employed with him. You can maintain ties with your mother without his knowledge and use subtle hints [or indirect speech] to avoid his anger. If you exert your utmost effort in being dutiful to your father and seeking his satisfaction, and he remains angry because of your maintaining ties with your mother, then we hope that his anger will not harm you.