I'm re-posting a message I sent a week or so ago:
This is paraphrased from George Way of Plean & Romilly Squire's "Scottish Clan & Family Encyclopedia" (HarperCollins & Barnes & Noble, 1994, 1998):
George Sinclair, 6th Earl of Caithness, was forced to sell estates in 1672 to settle debts. He died without issue in 1676 and Sir John Campbell of Glenorchy claimed the title, being in possession of the mortgaged estates. He also married the widowed Countess! George of Keiss was a descendant of the younger son of the 5th Earl of Caithness and disputed Campbell's claim. Thus the battle ensued on the banks of the Altimarlach near Wick where, it has been said, "so many Sinclairs were killed that the Campbells were able to cross the water without getting their feet wet."
So George 6th Earl died in 1676 and THEN Campbell claimed the estates. George of Keiss would have been related to George 6th through George 6th's father (the 5th Earl), and therefore he did have some legal standing on which to claim the estates.