Ever seen this ?

Ma

Ok so my niece is 4 years old and my husband caught her trying to bite my 4 month old nails off. Apparently she has been doing this for a while because she has a 8 month old sister and I found out today she was doing it to her too. I told her mom and dad (my brother) about it and he just rolled his eyes. I feel I shouldn’t have to punish her, But I mean that’s my son and she’s only a child..How would you handle this??