Do you think my niece's punishment is fair?

She's now 15 years old and has been acting out ever since she became a teenager.
So when I visited my sister and her over the other day she opened the door barefoot which already was a bit strange because I had never seen her without socks before. And not only that, she made a lot of housework by herself and she was unexpectedly kind. But her mother, a few hours earlier, didn't allow her to go for a walk with her friends. After she kindly offered me a chair to sit and made a favor for her mom, she told her something like: "Is this enough?" and her mom said: "Yes". Then she went in her room and she came back wearing her socks and slippers and with a smile in her face.
Later my sister told me that she found out her weakness that she hates her feet being seen and from now on, when she would do something bad, she would make her go barefoot until she feels really sorry about what she had done.
So what do you think about all that? Do you think it is a fair punishment or does it cross a line?
Do you think my niece's punishment is fair?
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