A Muslim woman does not have to wear hijaab in front
    of her non-Muslim mother’s husband
Question

                What is the ruling regarding wearing hijab in front of my mother's
      husband? Neither of them are Muslim, although Alhumdulilah, they are accepting of my being
      Muslim. They have no children together and married after I had already moved out of the
      house. Would he be considered my mahram or not?Juzak Allah kheir
Praise be to Allah.It is not obligatory because no report to that effect has been
    narrated, so we refer to the principle that all things are permitted except those which
    have been expressly forbidden. As far as a mahram ( a relative whom one is permanently
    forbidden to marry  this includes the mothers husband) for travelling is
    concerned, one of the conditions is that he should be a Muslim. And Allaah knows best.