The body of a woman who may have died as long ago as 2008 was discovered last week in the rear seat of a Jeep in the attached garage at Farrenkopf's home in Pontiac, 30 miles north of Detroit.
Paula Logan told the AP she feels "100 percent sure" it's her sister, but authorities haven't confirmed the identity yet.
Oakland County authorities have said Farrenkopf paid her mortgage and other bills automatically through a checking account that once had a balance of $54,000. The money ran out a year ago, and the home went into foreclosure. A contractor sent by the lender discovered the body.
Investigators are puzzled: There was no sign of foul play, and the key in the ignition was in the off position. The fuel tank had gas.
Paula Logan said her sister, one of 10 siblings from an Irish-Belgian family in the Boston area, typically stayed in touch even while working or traveling in Germany, England, Switzerland and Egypt. She said she last saw her in 2007.
"It's not like we were estranged. ... My sister had died. I tried to get in touch with her," Logan said. "My mother passed, too. We tried to get in touch with her. The phone just rang, rang, rang."
She said she asked police in Pontiac to go to Farrenkopf's home. She couldn't recall the date.
"They said they did a wellness check and didn't see anything wrong," Logan said.
Pontiac has been patrolled by the county sheriff's office since summer 2011. A message seeking comment was not immediately returned Tuesday.
Farrenkopf, who was single and would have turned 50 this year, would not have committed suicide, even if the auto industry's collapse in 2008 may have affected her work, Logan said.
"She was energetic, bubbly, well-rounded you know what I mean?" Logan said. "She had plenty of money. She had stocks, insurance. That house was a minute part of what she owned."
AP researcher Rhonda Shafner in New York contributed to this report.
Follow Ed White at http://twitter.com/edwhiteap