The family received a grant of the Barony of Roslin in Midlothian during the twelfth century. Through marriage, the family obtained the Earldom of Orkney, and in the next century received land in Caithness. The earldom or kingdom of Orkney was later resigned by order of James III.
The chief of clan Sinclair at this time split the family lands, giving the lands of Caithness to his elder son, and the lands at Roslin to his younger son. It was around this point that the spelling “Sinclair” came into general use, although the Earls of Roslin still prefer to use the older form of “St. Clare”.
The Earls of Caithness engaged in a long succession of feuds with their neighbours and within the family. This resulted in the 4th Earl imprisoning his son in Girnigoe Castle, where he starved to death.
Much of the Sinclair lands in Caithness had to be sold off to pay debts. Within the family, rights to estates were disputed and culminated in a battle between the Sinclairs and the Campbells on the banks of Altimarlech, near Wick. It is said that, as a result of this battle, “so many Sinclairs were killed that the Campbells were able to cross the water without getting their feet wet”.