What if my sister borrows money and doesn't pay it back?

Try not to let her pay back the money if it is not in urgent need. Think of it as putting it in the bank without interest. After all, family ties are more important than money. If the amount is not large, consider it as pocket money for my sister. If the amount is large, she will not take the initiative to repay it, and will never borrow it again, or let her lover or parents remind her.