Search to continue for missing SA trio

A search continues for three South Australians missing at sea but the family admits their loved ones are most likely dead.

Len and Annette Vandepeer and their adult son Doug failed to return to the Cape Jaffa boat ramp in SA's southeast on Monday night.

A three-day search has not found them or their seven-metre boat.

"We are being realistic and acknowledge that it would take a miracle to see our family alive now," Len's sister Sarah Vandepeer told reporters in Kingston on Thursday.

"Albeit we are still praying for that miracle."

On Friday the search will continue with aircraft, a helicopter and coastal patrols involved.

Police say they will assess its progress later in the day but expect to continue some form of search over the next few days.