Isabella Clara Eugenia's Wikipedia article has this, "When Albert died in 1621, Isabella joined the Third Order of St Francis and was appointed the Governor of the Netherlands on behalf of the King of Spain. She was succeeded as Governor by Cardinal-Infante Ferdinand, the third son of her half-brother Philip III of Spain in 1633." The Third Order of St. Francis includes both sexes of laity and lay clergy. So here we see her dressed vary plainly as a nun.
From Wikimedia.
Isabel is dressed as a nun in this Rubens portrait.
Keywords: Rubens, Infanta Isabel Clara Eugenia, Princess, Spanish, Habsburg family, veil, high enclosing neckline, nun's habit