On the one hand, it does seem unfair that you pay the price for your little sister's tardiness should she miss the bus. On the other hand, welcome to adulthood.
Besides, it's not like she's asking you to fly. I take my brother to school every day, and I work full time too.
But if you want to give your sister a little incentive, ask your mom if your sister can split the cost of gas should she miss the bus. That way, she'll at least learn a little responsibility.