Farzin Abdi is Sahel Kazemi's nephew, but the two were raised together like brother and sister.
Abdi says he does not believe Kazemi would have killed the former Tennessee Titans quarterback and herself.
The two were found dead on Saturday in a condominium owned by McNair.
Abdi says police told him they are almost sure the 20-year-old Kazemi was the shooter, but he says she had no motive.
Abdi says Kazemi believed McNair was divorcing his wife and she was preparing to sell her furniture to move in with him.
Nashville police didn't immediately have a reaction to Abdi's comments.
Police say McNair was the victim of homicide, though they aren't yet ready to label Sahel Kazemi's death a suicide.
Meanwhile, a public memorial and viewings are scheduled later this week for slain former NFL quarterback Steve McNair.
The public will have a couple of opportunities to attend viewings in Nashville on Thursday, and a memorial will be held later that evening at Mount Zion Baptist Church.
A funeral will be held Saturday in Mississippi but arrangements are not yet final. McNair, 36, was from Mt. Olive, a small town in southern Mississippi.
Fans are asked to make donations to the Steve McNair Foundation.