If they are travelling within their city no mahram is needed. But if they are traveling from one city to another, a Mahram is required. However, if they are going to travel without a Mahram whether with you or without you and they are properly covered, the lesser of the two evils is to take them with you.

I’ve sister & niece who study in the state my family lives. Both of them always travel to school without mahram. My question is can my husband & I give them lift whenever we travelled home?