According to her article in thepeerage, Countess Mary was a Campbell who married a Campbell, but after she married George Sinclair, 6th Earl of Caithness. Earl George passed in 1676. She married John Campbell of Glenorchy, 1st Earl of Breadalbane and Holland in 1678. She had one child only - by him. He had children by an association with one Mildred Littler after Lady Mary died in 1691 and also by his previous marriage to Lady Mary Rich in 1657. John Campbell was a consummate scoundrel according to articles in Wikipedia and thepeerage.
From the Sotheby's Web site.