John Williamson Sinclair

Was living in Norway as a hostage for his brother Henrik from 1379 to 1380, and is said to have married Ingeborg, a natural daughter of Waldemar, King of Denmark during this time.

Source:
The Saint-Clairs of the Isles; being a history of the sea-kings of Orkney and their Scottish successors of the sirname of Sinclair, Roland William Saint-Clair (Published 1898), page 96
https://archive.org/details/saintclairsofisl00sain/page/96/mode/2up

"The reversal concerning Orkney not being found sufficient by King Hakon, the ambassadors were allowed to stay in the city of Tunsberg, in Norway, till His Majesty was satisfied . In the meantime there was a marriage concluded, as is said, betwixt John Saint-Clair, brother to the Earl, and Ingeberg, natural daughter of Waldemar, King of Denmark, by Jova Little, who was a daughter of Sir John Little, Commissioner of Rugen."