There can be several reasons for a child that age to (Take things,that don't belong to her.I should hope, your sister doesn't refer to her daughter as a thief or stealer. If she punished right away, rather than calmly explaining to her how inapropriate the act is,then she may continue to take things,out of spite. This should have been taken care of (Calmly) I don't know your sister,or if she is a SAHM or works,but alot of times, children will act out like this, when they feel neglected or insecure.They find instant gratification,when they find something pretty,and so she probably is being truthful with her mom, as far as the reason she takes things. Alot of times they will do things like this, when something has changed in their life, like a move, or a new school, New boyfriend or a divorce.Why don't you suggest to your sister,that she sit her daughter down,and (Teach By example)explain how it can hurt your feelings if someone took your favorite doll? or favorite shoes?or favorite necklace that someone dear gave her.Tell her, if she finds something pretty or that she likes,to ask mom if she could get one like it.She needs to know, that asking is the way,not taking,Mom needs to also do some soul searching, and try to figure out, why her daughter is feeling so insecure right now.The best to your sister,and her daughter