George Vertue. Notebook entry. 1731 [published in "Vertue Note Books Volume IV" in Walpole Society 24 (1935–36), p. 17], mentions a half-length portrait of two ladies which he saw on a visit to Cassiobury in 1731—apparently this picture.
Hermann Warner Williams Jr. "The Bequest of Jacob Ruppert." Metropolitan Museum of Art Bulletin 34 (July 1939), pp. 166–67, ill., dates it shortly after 1660.
R. B. Beckett. Letter. April 21, 1948 [letter unlocated; transcript only], considers the Capel portraits "of course, very good".
R. B. Beckett. Letter. December 24, 1948 [letter unlocated; transcript only], dates it about 1660.
R. B. Beckett. Lely. London, 1951, pp. 12–13, 38, 70, no. 63, pl. 36, dates it soon after the marriage of Elizabeth, the younger sister, in about 1653.
Margaret Whinney and Oliver Millar. English Art, 1625-1714. Oxford, 1957, p. 281, observe that one of Elizabeth Capel's flower studies, signed and dated 1662, survives at Windsor.
Oliver Millar. The Tudor, Stuart and Early Georgian Pictures in the Collection of Her Majesty the Queen. London, 1963, text vol., p. 138, notes that there is a similarly signed flower study by the Countess in the Royal Collection (Kew; Hampton Court no. 1465).
Oliver Millar. Sir Peter Lely, 1618–80. Exh. cat., National Portrait Gallery. London, 1978, pp. 50–51, no. 27, ill., describes this painting and the double portrait of Lord and Lady Capel at the Yale Center for British Art [now in the National Portrait Gallery, London] as the finest in the set of Lely's Capel family portraits, and suggests a probable date of about 1658.
Christopher Brown. Van Dyck. Ithaca, N.Y., 1982, pp. 122–23, pl. 230, describes it as "clearly based on Van Dyckian models".
Zirka Zaremba Filipczak. "Reflections on Motifs in Van Dyck's Portraits." Anthony van Dyck. Exh. cat., National Gallery of Art. Washington, 1990, p. 64, suggests that Lely, although influenced by Van Dyck's "Dorothy Savage, Viscountess Andover, and Her Sister, Elizabeth, Lady Thimbleby" (National Gallery, London), returns to double portraits that simply juxtapose two sisters, as in our painting.
Richard Charlton-Jones. "Lely to Kneller, 1650–1723." The British Portrait, 1660–1960. Woodbridge, England, 1991, p. 81, observes that in our portrait the model is still Van Dyck but "the smooth, even finish . . . of the earlier 1650s [is] giving way to a more confident, open treatment".
Christopher Baker and Tom Henry. The National Gallery: Complete Illustrated Catalogue. London, 1995, p. 212.
Katharine Baetjer. "British Portraits in The Metropolitan Museum of Art." Metropolitan Museum of Art Bulletin 57 (Summer 1999), pp. 14, 17–20, ill. (color, overall and details), mentions that Elizabeth's identity is secured by the display of her own signed flower painting and that Mary's lack of a widow's veil indicates that she was painted after 1657, when she was already married to her second husband, Lord Seymour, later first Duke of Beaufort.
Sue Bennett. Five Centuries of Women & Gardens. Exh. cat., National Portrait Gallery. London, 2000, p. 37, ill. (color).
Julia Marciari Alexander in Great British Paintings from American Collections: Holbein to Hockney. Exh. cat., Yale Center for British Art, Yale University. New Haven, 2001, pp. 55–57, no. 4, ill. (color), says the portrait epitomizes Lely's best work from the 1650s and early 1660s, and finds that it captures the individuality of his sitters and is "one of the finest Lelys currently in an American collection".
Katharine Baetjer. British Paintings in The Metropolitan Museum of Art, 1575–1875. New York, 2009, pp. 17–20, no. 7, ill. (color).
Old Master & British Paintings. Christie's, London. July 3, 2012, p. 240, under no. 58.