Portrait of Elizabeth and Philadelphia Wharton

The date is written on the picture itself. Along the bottom edge an old inscription names the two girls as Philadelphia and Elizabeth Wharton, the only daughters of Philip, Lord Wharton, by his first wife, gives the year as 1640, and puts them at about four and five years old.

That year is the point. Their father, Philip, fourth Baron Wharton, was one of the peers who sided with Parliament against Charles I. In 1642 he was made lord lieutenant of Buckinghamshire and Lancashire and took a regiment of foot into the field at Edgehill. The man painting his daughters was the king's own portraitist, working in London two years before the two of them ended up on opposite sides of a civil war.

Wharton kept his van Dycks at Winchendon in Buckinghamshire. His grandson sold them in 1725 to Robert Walpole, who hung them at Houghton Hall in Norfolk, and in 1779 Walpole's grandson sold the whole Houghton collection to Catherine the Great. The sisters have hung in St Petersburg ever since, between the green curtain and the strip of open landscape van Dyck gave them.