Strange but true: Your young child is naughtier when she is with you. Why doesn’t she “bully” other people?
For starters, there is the novelty factor. The relative unfamiliarity of another person can have a very stabilising effect on a kid’s behaviour. For example, if your aunt cared for your child every day, her excitement at being with her would soon wear off. You can be sure she’ll misbehave with her occasionally, as well.