What are the limits of the relationship between my son-in-law and my second wife? Is he a permanent or a temporary Mahram (unmarriageable relative) for her, or is he not a Mahram at all?
Please enlighten me, and may Allah grant you knowledge.
Your son-in-law is not a Mahram for your second wife. It is not permissible for him to be alone with her or to travel with her like any other non-Mahram. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.