How do i tell my sister I need to live my own life I can't always be with her?

So my sister is 23 and I'm 16 the thing is I helped my sis when she had her first child 3 years ago I helped her clean I cooked but now alls she does is call me to help her I understand she's not a good cleaner and can't cook but sometimes I want to hang out with friends. She has me cook breakfast in the morning (I don't live with her but sometimes I'll go in the morning about 9 because she calls and sometimes I sleep over to make it easier) I clean I take care of her son take him to the park feed him drop him off to day care pick him up take him home and he's mines until my sister gets home. There was a time where she called me told me she was going to take me out for fun and she was going to leave her son with her husband but it ended up being that she left me in the house with the baby and left with her husband. (there was a time I stoped answering her phone calls cuz I didn't want to go). I really care for the baby I love him and if I leave it to my sister her sin will starve and be live in a dirty insannitary place. My mom says she's not going to help her because shell never learn and she doesn't want to commit the same mistake my aunt did, she took care of her daughters daughter and then the rest followed she took care of her childrens kids and is still taking care of them she is 48