Murray Olds: Australian correspondent on the NSW 17-year-old who stabbed her sister to death - Heather du Plessis-Allan Drive

A 17-year-old girl has shocked NSW authorities by stabbing her younger sister to death.

Emergency services were called to a Boolaroo address to address a stabbing that occurred at 3:45pm on Monday.

Australian correspondent Murray Olds says police were mystified by this case - and haven't uncovered a motive.

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