The fact that you would do this suggests that you're immature, irresponsible, reckless, and selfish. You have to grow up and decide what's more important to you. Money, or your family.
I understand that you're probably too young to get a proper job and pay them back, but you got yourself into this situation, what can you expect? I don't think you'd be very pleased if your sister stole 50$ from you, and then your grandma went and took your pot.
Even if it takes you a year and you think they've forgotten, you should still pay them back what you owe (which may restore their trust in you).
I understand that you're probably too young to get a proper job and pay them back, but you got yourself into this situation, what can you expect? I don't think you'd be very pleased if your sister stole 50$ from you, and then your grandma went and took your pot.
Even if it takes you a year and you think they've forgotten, you should still pay them back what you owe (which may restore their trust in you).