This certainly shouldn't be taken lightly as it concern safety of the children. I would suggest bringing it up to the authority as well so that it could be highlighted. Regardless how busy the teacher is, a child should not be left unattended. More so during such timings where caregivers are coming to fetch their children. Every child should be accounted for and only released to their respective caregivers.
Hope your niece (and your sister!) did not suffer any shock from this...
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