These Ladies were sisters. Lady Frances' article from thepeerage.com comes first, "Lady Frances Stewart was born on 19 March 1617. She married Jerome Weston, 2nd Earl of Portland, son of Richard Weston, 1st Earl of Portland and Frances Waldegrave, on 10 June 1632 at Roehampton, Surrey, England. She died on 13 March 1693/94 at age 76. She was buried on 24 March 1693/94 at Westminster Abbey, Westminster, London, England.
She was the daughter of Esmé Stuart, 3rd Duke of Lennox and Katherine Clifton, Baroness Clifton (of Leighton Bromswold). Her married name became Weston."
This is Lady Katheine's brief article from thepeerage.com, "Lady Katherine Howard married, firstly, George Stuart, 9th Seigneur d'Aubigny, son of Esmé Stuart, 3rd Duke of Lennox and Katherine Clifton, Baroness Clifton (of Leighton Bromswold), in 1638 in a secret marriage. She married, secondly, James Livingston, 1st Earl of Newburgh before 1649. She died in 1650 at The Hague, The Netherlands, in exile.
She was the daughter of Theophilus Howard, 2nd Earl of Suffolk and Lady Elizabeth Home. From 1638, her married name became Stuart. From before 1649, her married name became Livingston."