The former Port Lincoln resident still holds out hope of news about what happened to Jarred, then 28, in 1999, after he was last seen near his father's Port Lincoln home.
"There's always hope, there's a hope of a result," said Toni Mueller, his older sister.
"But it's not necessarily the hope that he's going to be found alive and well."
The Mueller family have been given new hope of finding out what happened, thanks to Australian Federal Police age progression techniques, in which help create images of what long-term missing persons may look like today.
The image of what Jarred may look like at 42 was produced by forensic artists from the Australian Federal Police and has been featured on a national poster along with age progressed images of other long-term missing persons.
"It was amazing to see him, I just saw my dad strait away," Ms Mueller said.
"I look at it and think, this is exactly what he would look like. It's great to have it on the posters that somebody might recognise him."
Jarred disappeared after going to his father's home for help when he ran out of petrol near Pt Lincoln.
"Dad offered to give him a hand to get the car started again and Jarred wasn't ready to do it at that particular time and dad said to him 'let me know when you're ready'.
"Dad was working from home so he went to do some work...when he came back out Jarred was gone." Jarred's dad went to the car, put petrol in it and got it started.
"Two days later, the car had been moved but Jarred was never seen again," Ms Mueller said.
Ms Mueller appealed for anyone who thinks they may have seen Jarred to contact Missing Persons on 1800 000 634.
PORT Lincoln man Jarred Mueller is being featured in a Missing Persons Week promotion this week.
Mr Mueller has been missing since June 2, 1999.
Australian Federal Police renewed the campaign to find Mr Mueller in June, releasing an age progressed poster of Mr Mueller showing what he looked like at the time he went missing and what he would look like now.
His sister Toni Mueller, who now lives in Armidale, attended the Missing Persons Week launches in Canberra and Adelaide to help raise awareness about her brother.
She said while he remained missing, some minor information had been received since the poster was released that had been helpful in trying to piece together the puzzle.
"There's been people talking in Port Lincoln and it comes back to us and we pass it on to the police," she said.
"There's been small bits of information that could lead to something, but even if not, it's just good to have the information."
She said it was likely there were people who knew more who were not coming forward.
"Lots of people don't want to get involved; they'll tell someone what they know but they won't tell the police.
"But they shouldn't be worried about coming forward because Crimestoppers is there for people to make anonymous phone calls."
On the day Mr Mueller went missing he was seen at his father's house in the morning and was due to attend his daughter's birthday party in the afternoon, but he never arrived.
If you have any information contact police on 131 444 or 8688 3020, or if you wish to remain anonymous call Crimestoppers on 1800 333 000.