The witness, who has name suppression, said the family visited from China for a relative's wedding at that time.
Later, Preston returned to New Zealand for his mother's funeral, with Ms Fan's two children.
The witness said she understood Ms Fan, a Chinese national, wanted to come to New Zealand, but Preston at first refused to sign the requisite "paperwork".
The Court earlier heard the couple met in China in 2003, had two children, and separated in 2011.
On returning to New Zealand, Preston stayed in a hostel, later moving to Salvation Army accommodation, the witness added.
The relative said Preston told her Ms Fan wanted to get full custody of the children.
Crown prosecutor Grant Burston asked if Preston tended to focus on this topic, and on his concerns about Ms Fan.
"That's all he ever talked about," she said. "[It was] depressing and stressful because it was just obsessive - it was never a normal conversation."
After Ms Fan was found dead, a member of Victim Support discussed with the witness how to break the news to Ms Fans' children.
The witness, at times clearly distraught, said it was decided the best way was to be frank, and tell the children their mum was dead.
Sergeant Gregory Simmons was working at Wellington Police Criminal Investigation Branch when Ms Fan was killed.
On 11 November 2013, a day after Ms Fan was found dead, Mr Simmons was the officer in charge of the scene at Brussels St, Miramar.
"There was blood on the exterior of the laundry door - both on the frame and on the glass."
The Court earlier heard Ms Fan likely died from blood loss after being stabbed.
The Crown case is that Preston killed his wife to gain sole custody of the two children.
The trial before Justice Joseph Williams and a jury of twelve continues.
