Nat Vickers wrote: “My brother in law Mark VIckers has been missing from the morning of Friday 4 March, leaving Intake he has not been seen since."
The family say there have been possible sightings of him in Balby and Rotherham town centre.
She added: “The police are also out looking for him and my husband has been out day and night also. Mark is very vulnerable and needs finding ASAP.
“Any info please message me.”
South Yorkshire Police have not issued any official missing person appeals for Mark.
Anyone with information on his whereabouts can contact the family via www.facebook.com/nat.pearsall or call police on 101 or Crime Stoppers anonymously on 0800 555 111.