A 21-year-old woman, who cannot be named for legal reasons, confronted a 15-year-old girl outside the Wollongong Youth Centre on April 28 and accused her of leaving red 'hickey' marks on the boy's neck.

The 21-year-old, who is the boy's sister, yelled at the teen for several minutes before taking hold of her clothing and dragging her off her seat, then pushing her back onto it.

With her face just inches from the teen's face, the older woman continued to berate the teen and repeatedly slapped her in the head.

She eventually let go of the victim and they parted ways as the woman said "you dumb slut, if you go near my brother again I will hurt you".

Meanwhile, a witnesses contacted triple zero while two members of the public approached the woman.

She quickly told them to "f--k off".

Police caught up with the woman about 50 metres down the road and she was arrested.

She told police she was upset with the victim because she had been giving her 11-year-old brother 'hickeys' on his neck.

The woman pleaded guilty to affray in Wollongong Local Court on Tuesday, as well as an unrelated charge of shoplifting stemming from the theft of nearly $500 worth of pre-mixed spirits from Dan Murphys.

The court heard the woman and a friend took an empty pram into the store, filled it up with drinks and placed a towel over the opening before walking out of the store without paying.

She will face sentencing on both matters in August.