Advice regarding giving a co-worker rides to and from work?

A new girl was just hired. She’s family to two people I work with and she usually rides with them. Today her aunt (yet another family member in a different but related department) asked me if I could take her home, since I live 3 miles away.

This may seem selfish, but I don’t want this to become a regular occurrence. I don’t want to be responsible for someone getting to work, nor do I want to possibly be late because I was picking up someone else. I work a lot of overtime and don’t have much time to myself, and I don’t want what little time I have being cut into by giving this girl rides. The two people I work with, her family members, work slightly different shifts so it requires this girl to come in early, or them to stay late. How do I avoid being roped into being a regular ride?