Do i have to baby sit my siblings?

Im 15 years old. I like my independence. Yet it seems like everytime i have the chance to get out my Mother makes me babysit. I have to baby sit everyday after school from 3:00 to 5. Then i get to baby sit on wednesday and fridays cuz my sister had basketball games. Today i wanted to go shopping with my friend( with my money, andi had rides) and she said no you have to go to work for me....( she owns a coffee bar and it make like 20.00 a day so i get paid like 10 bucks from 9am until 4:30 plus tips....) This morning comes around and i felt like CRAP i had a migrane which i get alot. So i told her that So she got pissed off and said fine then since your not leaving today then you can babysit. So she called gma and said i will babysit so no need for her to now. So i had to now instead of working babysit my siblings for free. Still not feeling good. Is she aloowed to p her kids off on me. Or make me work her job for less than minimun wage??