His first cousin-once-removed was George Sinclair, 6th Earl of Caithness (d. 1676) who was the son of John Sinclair, Master of Berriedale and his wife Jean, daughter of Colin Mackenzie, 1st Earl of Seaforth.

[1] George Sinclair, 6th Earl of Caithness married Mary, daughter of Archibald Campbell, 1st Marquess of Argyll, but had no children.

The Earldom of Caithness being much in debt, he had transferred the estates and the title to his principal creditor, John Campbell of Glenorchy, who upon Sinclair's death was created Earl of Caithness by patent.

Inflamed with drink, the Sinclairs attacked the force of Campbells and were routed in what is known as the Battle of Altimarlach.

Legend has it that so many Sinclairs were killed that the Campbells were able to cross the river without getting their feet wet.