I'm going to steal my sister's koi fish and you can't stop me
Why do you want to steal your sister's koi fish?
I'm going to give out these koi fishes to my friends instead
Those are you sister's fishes and you are a horrible sibling for trying to take away her joy. That is her property and you have no right stealing it for your own benefit. Do you want to see your sister cry and resent you?
Maybe I do. I'm going to do it anyway since she is going to be angry
You can stop your actions now and return them. Even if she will be upset that you took them in the first place, it would only be worse if you gave them away. It's not worth it.