A woman who claims former world boxing champion Eamonn Magee tried to throw her downstairs "would go mad with drink in her," a court has been told.
Mr Magee's ex-partner Maria Magill also repeatedly went back to him despite vowing to end their relationship, the court heard.
Mr Magee, 38, of Eskdale Gardens, Belfast, denies assault causing actual bodily harm to Ms Magill.
Mr Magee's sister-in-law Phyllis made the comments about Ms Magill on Monday.
Previously, Ms Magill had claimed Mr Magee attacked her last September after the couple had been out drinking.
She claimed when they went back to a flat, he asked her to get into bed and then grabbed her by the hair.
She said he trailed her out of the room, tried to throw her downstairs and filmed her on a mobile phone.
She said she suffered cuts, bruising and lost some hair as a result.
But at Belfast Magistrate's Court on Monday, Phyllis Magee claimed Ms Magill said " stupid things" to her.
"She always told me she wouldn't go back to Eamonn, but she always went back to him," she said.
Mrs Magee recalled spending the day of the alleged assault drinking with Ms Magill in a working man's club.
"She loves drink... she would drink nearly every night," the court heard.
The boxer's sister-in-law agreed with a suggestion by defence counsel that Ms Magill behaved erratically.
"She just would go mad with drink in her. She would do things with drink in her that she wouldn't when she was sober," she claimed.
Mrs Magee said if her brother-in-law phoned Ms Magill she would go into another room to take the call.
"You would always hear her cursing, saying 'Don't be phoning me again', then next thing she would be sitting texting him," she told the court.
During the hearing it was alleged the couple's relationship overlapped with a period where Mr Magee was still with his ex-wife Mary.
Phyllis Magee said the two women "hated each other" but then formed a close friendship.
A prosecuting lawyer suggested that might have developed from looking after each other's children.
"We know the reason why they might not have liked each other in the first place," the lawyer said.
"But do you accept why it is possible when looking after each other's children they forged a friendship. It's actually not unusual."
But Mrs Magee replied: "Maria told me to keep your friends close but your enemies closer. She always told me that."
The court also heard the boxer claimed he received a text message from Ms Magill last week.
A police officer who investigated the assault allegations declined to comment when asked if that would be inconsistent with her evidence.
Mr Quinn also put it to him: "Are you aware of Mr Magee's contention that there is a degree of collusion going on here between his former partners, Maria Magill and Mary Magee?"
The officer said no investigation was ongoing into those allegations.
Mr Magee is expected to give evidence when the hearing resumes at a later date.