Sundstein was the son of Johanna Malena (born Jensen) and Hans Jacob Matras Sundstein from Tórshavn.

He was married to Lydia Marsten (born 20 November 1927[1]) from Klaksvík.

Sundstein was a certified accountant and was a member of numerous boards of directors.

In 2008 Sundstein published his memoirs, becoming the first Faroese politician to do so.

[3] Sundstein died on 8 July 2024, at the age of 91.