My sister gave me half of the money for my wedding dress and I'm not going to pay her back!
Are you going to keep it?
Yes, of course I am not going to give it back to her! I like it.
You shouldn't renege on your debts. You should pay what you owe to your sister.
I may still pay her back.
You should 100% pay her back, as she was kind enough to pay for half of your dress. Imagine if you did something kind by loaning someone money and then they ran away with it? Now imagine if that was your own family!