Is a non-Muslim brother, father, etc. a mahram for their Muslim sister / daughter?

Answer

Non-Muslim family members cannot act as mahrams for Muslim females. The objective for having a mahram is to guard the honour, modesty and chastity of the woman. This objective is not attainable from a non-Muslim relative. In fact, a faasiq father, brother, uncle, etc. also are not valid mahrams for their females, especially for a journey despite them being Muslims.