Hinemoa Epere said she left when things "got out of control", but her brother and mother remained and hit Ms Stewart with a baseball bat they had found at the house, the court heard.
Ms Stewart's partner fled to seek help, but a lodger and Ms Stewart's 10-year-old daughter remained and witnessed the attack, leaving the young girl traumatised.
Ms Stewart suffered a fractured and dislocated elbow, a broken knee and a deep laceration to the back of her head that required stapling to repair. She was in hospital for three days and still suffers from the attack's after-effects, the court heard.
Horeta Epere's lawyer Vicki Pomeroy said her client hit Ms Stewart a couple of times and then left the room as Albert continued the attack. Horeta Epere administered the blows after hearing that Ms Stewart had assaulted Albert, who did not report the matter to police.
Justice Murray Gilbert rejected the idea of provocation but accepted that Albert had instigated the attack.
Hinemoa Epere's lawyer Nathan Speir said his client had a drinking problem.
"The number of alcoholic drinks consumed on the night in question is extraordinary for one so young, and concerning. The [pre-sentence] report writer notes it appears Ms Epere has had this problem since she was 11-years-old."
She admitted two charges of burglary and was jailed today for one year and 11 months.
Horeta Epere was sentenced today to seven years and two months in jail after admitting one charge each of burglary and causing grievous bodily harm with intent.
Albert had previously been sentenced to seven years and eight months' jail on two charges of burglary and one of causing grievous bodily harm with intent.
