Ann Allan said the disappearance of 28-year-old Samuel Townsley had left a "gaping void" in their family's heart.
The last confirmed sighting of him was at Union Street in Glasgow at about 09:30 on Wednesday 19 October 2016.
Mrs Allan said she was praying for his safe return and assured him that he was not in any trouble.
Speaking during a Police Scotland news conference, Mrs Allan said of her brother: "He is a son, grandson, brother, nephew, cousin, uncle and friend and his disappearance has left a gaping void in all our hearts.
"If you are in contact with Samuel, please let him know we only want him to get in touch so that we know he is safe and well.
"He is in no trouble at all and he is dearly loved and needed by those who are looking for him."
Mrs Allan pleaded with her brother to come home should he hear the appeal.
She added: "We will get you any help you need and look after you. You are not alone. You are sadly missed and I am sorry that you have suffered so much. You will never be on your own again."
Officers said they were liaising with the British Transport Police and homelessness charities in their search for Mr Townsley. They have also placed posters appealing for information in Clydebank and Dumbarton.
Samuel Townsley is described as being about 5ft 3in tall, with a stocky build, short brown hair and scars on his forehead. Officers believe he may have changed his appearance since he was last seen.
He was last spotted in Glasgow's Union Street, however, police have since received a report that he may have visited the city's Marie Trust in Midland Street later the same day.
Other possible sightings include Cathcart Road about seven or ten days before Christmas and a possible sighting at Clydebank Job Centre between 4-11 January 2017.
Mr Townsely is also known to visit places including Renfrewshire, Lanarkshire as well as Grangemouth and Falkirk.
Ch Insp Brian Shaw said: "While Samuel isn't always in regular contact with his family he does eventually get in touch.
"This time has been different and no-one has had any contact from him for a considerable length of time.
"His family is, understandably, very worried and just want to know he is ok."
A woman has made an emotional appeal for information about the fate of her brother, who has not been seen for four months.