Can a disbeliever be regarded as a mahram for his Muslim sister when travelling?

Is the son of a half-sister regarded as a mahram?

The sister of the wife’s grandfather and the sister of her grandmother are forbidden for the husband in marriage as her paternal and maternal aunt are forbidden for him

Is my father a mahram to the wife of my half-brother through my mother?

Is My Husband’s Uncle A Mahram to Me?

The ruling on castration

The father’s (ex-)wife is a mahram to his sons even if she did not bear him any children

Is The Husband’s Grandfather a Mahram to the Wife?

Is the Mother’s Uncle a Mahram? 

Her husband divorced her; is his father still a mahram to her?