Was your sister and your aunts part of the "they" that encouraged you to come home? Of course I don't know the whole situation, however, I would agree with your sister on this one. She helped you today by watching your kids, her children and her helped you over the summer, maybe she feels she's helped more than enough. Have you reciprocated? Have you watched her kids, run errands for her, cleaned her house? Or most importantly shown her that you truely appreciate her help? If you're only asking her to watch your kids when you go on job interviews, then you must be expecting her to be "on call", because employers rarely give you more than 24 hours when they call you to set up an interview.
You mention the children's father (your not married) is not with you. What does that have to do with anything? He should be giving you money to support HIS children. At least enough for you to hire a babysitter for a few hours a week.
P.S. Go give mom a giant hug, tell her how much you love her, thank her again for supporting you and your children, and over the weekend scrub the house from top to bottom.