Aunt always asking me to babysit?

Im 14, and my family lives a state away, but late May my aunt&her fam moved in our neighborhood. Im glad and all and love my cousins. But she's constantly asking me to babysit now. She has 3 kids but the eldest boy(13)wont watch them, so I watch the two boys who are 9&6. I dont mind watching them, even though she pays me less since we're family. For ex every Friday she's dropped them off without calling, and I watch them from around 5pm-9pm or so for $15. But if she needs me quicky ill just watch them for free as they are family ex for when she just wants to go to the mall by herself, grocery shopping, dr appt etc. I don't mind it but shes had me babysit them pretty much every single day this week. I don't want to be rude since she's family but id like some just relax time too. What should I do?