Q: A Muslim woman has a non Muslim brother. Is she allowed to travel with him?
A: A Muslim lady cannot travel the distance of 77 km or more with a non Muslim brother. Among the conditions of a mehram for a lady intending to travel such a distance is that he should be a Muslim. A Non-Muslim cannot serve as a mehram. (Raddul-Muhtaar)
And Allah Ta’ala knows best
Mufti Siraj Desai