Lady Mary's sister, Lady Frances, had, on March 8, 1712, married John, second Baron Gower, who afterwards was created Earl Gower.

Lady Mary's other sister, Lady Evelyn, on July 26, 1714, became the second wife of John Erskine, sixth or eleventh Earl of Mar of the Erskine line, who presently came into prominence as an adherent of the Pretender in the rebellion of '15, after which he fled the country.

He was created Duke of Mar by the Pretender.

Finally, the Marquess of Dorchester, being then in his fiftieth year, took for his second wife, on August 2, 1714, Lady Isabella Bentinck, fifth daughter of William, first Earl of Portland and his first wife, Anne, sister of Edward, first Earl of Jersey.

There was issue of this marriage two daughters: Caroline, who married Thomas Brand, of Kempton, Hertfordshire; and Anne, who died unmarried in 1739 at the age of twenty.
Already, on July 1, 1723, had died Lord Dorchester's only son and heir, William, who took the style of Earl of Kingston.