Tangvald set off from French Guiana on March 4th, aiming to make his way to the island of Fernando de Noronha, off Brazil. But he never arrived.
His sister Virginia Tangvald (27), on Monday told VG, that she hoped that the French and American governments would continue the search for her brother, after the Coast Guard in French Guiana called off he search last week.
"I hope the French and American governments can carry out further searches for Thomas," she told VG newspaper on Monday.
Tangvald's father Per Tangvald set off on his yacht "Dorothea" aged 25 in 1959, and continued sailing around the world until he died in a shipwreck in 1991.
His son Thomas, then 15 years old, was the only survivor of the accident, which killed both his father and his sister Carmen.
His mother Lydia, Per Tangvald's fifth wife, was shot by pirates when he was just three years old.
"Our family has experienced many tragedies," Virginia told VG. "But when you put yourself on the quest for freedom and finding one's true nature, there is no other way of living that does not feel like a tragedy in itself. That is the legacy Per Tangvald."
She said that the waters around the mouth of the Amazon River, where her brother has disappeared are unusually treacherous, with a lot of detritus washed downstream.
Tangvald had aimed to settle down with his wife Christina and their two sons Gaston and Lucio on the island, where he intended to continue his work as a naval architect.
Jacques Mertens, Thomas's step-grandfather, told yachting website Wavetrain that he had not yet given up hope.
"Normally I would consider him lost, but Thomas is a very special person," Jacques wrote in an email. "I still have some hope that Thomas is anchored somewhere drinking cocktails and will reappear to surprise everybody."
Here's a YouTube video of Thomas Tangvald sailing in the Caribbean on his converted Puerto Rican sloop Oasis: