I hate my summer job, what should I do?

My sister offered me a job at her work, and I have been working here for 3 weeks now and I absolutely hate it. My hours are not set because I am a "fill-in" so I am just placed where I am needed and that is something I was not aware of going into the job.

The only set hours I have are from 7am-9am every day, which is fine but the rest of my hours are all over the place. Some days those are my only hours, and other days I will have a 10+ hour day. And the WORST PART of all this is that I do ABSOLUTELY NOTHING.

The work I do is with special needs adults. Basically we teach the adults life skills on how to live alone. This requires me to drive all over the place which is putting extreme wear and tear on my car. Also, this is no where near my field of study (Environmental Engineering) and I am very uncomfortable with the "work" that I do.

I feel like my summer is being blown away and the only thing I am getting out of this job is frustration and a little bit of money.

What should I do? Should I tell my sister? She does the scheduling so I think she can help alter my schedule so that it is bearable.