<human>: The title says the cut and dry. 

I am a nanny during the week Mon-Thurs. My sister works at a cupcake place and mentioned they may need someone to just fill in when other people need to take days off. Exactly a week ago, I went to her workplace to talk to the boss and get more info about the job. She loved me and when I left said "you wouldn't want to start this Saturday right?" To which I said "No I would rather not". In my head I meant to say "I still need time to decide" but in reality I ended up walking out with all the paperwork I needed to fill out so I could begin work. 

The job would be mostly Fridays and Saturdays making my three day weekend that I have been looking forward to all year turn into a one day weekend. I am gone for 13 out of the 21 days they would have me work for. 

Now, a week later, I have emailed  explaining how much time I will have to miss for already planned events. I am stressing because I am 18 and my girlfriend and I want to go camping and stuff but we don't have solid plans but I wouldn't be able to get any more days off. 

I feel like I accepted the job but now I am physically sick over the situation because I don't want to leave a bad mark for my sister's sake but I also feel like this is not a good job for me to take. 

How do I professionally email them again and say that I am not going to be able to take the job?

Condense the text into a few key points
<bot>: I went to my sister's workplace and ended up leaving with a job. I am supposed to start/train Saturday but I was offered/took the job on a hectic day and regret my decision. What do I do?