I'm a regular user writing this post on behalf of my dsis and dn (hence the name change 
Dn is 5, bright as a button and generally lovely young girl. She has had a habit in the past of taking things belonged to other kids if she really wanted them etc but always small isolated incidents. Lately however this behaviour has really escalated and dsis is tearing her hair out.
Recent examples, on Friday in a shop she noticed dn put something behind her back. It turned out she was hiding a creme egg, which she hadn't even asked dsis for. Dsis explained that this was stealing and was wrong / serious and made her put it back. They were visiting us for the weekend and when dsis went to pack up stuff dn told her multiple times there was no need to check her bag as it was all sorted. When dsis checked her bag she had taken two things belonging to my dd, one of which was a special something related to her birthday activity that weekend.
Dsis had a big chat with her on the way home about how people get in lots of trouble for that kind of behaviour and adults go to jail etc and why would she behave like that. She felt like she had gotten through to db.
Then dn came home from school today and refused to hand over book bag and threw a massive tantrum when dsis took it and opened it. Today, she has taken 11 little animals from school. She clearly knows this is wrong but is continuing...
What to do? Dsis has rang a neighbour whose DH is in the police and asked him to come have a chat with her. Has anyone any advice on how best to tackle the issue?
Thanks