Ewen Macdonald, 32, is on trial for the murder of Mr Guy, who the Crown says was killed amid tensions over the future of the the family farm in Feilding.
Macdonald has denied shooting dead Mr Guy, 31, in the driveway of his home early on July 8, 2010.
Giving evidence in the High Court at Wellington today, Mr Guy's older sister Nikki Guy said she had noticed differences between Macdonald and her brother.
The two men co-managed the business together after Mr Guy returned to the farm after living in Hawkes Bay.
"When Scott came back they were both equals but the farm wasn't big enough for both of them to have the same, equal roles," Ms Guy said.
She said there were also personality differences.
Her brother Scott was all about the "big picture" while Macdonald was more task-oriented.
Ms Guy also spoke about an incident at the fifth anniversary of the business she owned with mother Jo.
Macdonald made a comment to Mr Guy, who subsequently left with wife Kylee.
Ms Guy said Macdonald did not seem concerned.
"He was just sitting there with his arms crossed and he didn't look too worried or too concerned that ... he'd ruined our dinner party."
Ms Guy also gave evidence that she initially thought Mr Guy had burnt down an old house on his Aorangi Rd property, which was up the road from the family farm.
The Crown alleges Macdonald burnt down the house and later vandalised a new house that was under construction to scare Kylee Guy off the farm.
Ms Guy said Scott Guy was frustrated the house had not been shifted off quickly enough and wanted to get on with building a new home.
"He said if it's not moved shortly I'll burn it down myself."
Ms Guy said he and Kylee were "terrorised" by the later vandalism of the new house and Kylee had reservations about moving in.
Ms Guy said her brother Scott was "on the war path" and wanted to catch whoever had done it.
He and younger brother Callum stayed the night in case the vandals came back, she said.
Ms Guy said that before his death, she thought Mr Guy was looking forward to moving on and working with Macdonald in the future.
"I noticed that the last time I talked to him he didn't say anything about Ewen at all."
Under cross-examination by defence lawyer Greg King, Ms Guy said she had not heard of offensive notes being left at Scott and Kylee's property before his death.
Members of the Guy family are among the Crown's key witnesses and will take to the stand on more than one occasion throughout the trial, which is now in its second week.
The court is this week hearing evidence about the relationship between Macdonald and Mr Guy, which is crucial in establishing a motive.
Mr Guy's mother Jo this morning gave evidence about tensions between the two men.
