Sofonisba Anguissola, 1551, Portrait of a Nun
From the Web Gallery of Art:

In the painting the nun wears a lily-white habit which stands out against the dark background. The red and gold bound book of prayers offers the only source of detail. The sitter is usually identified as Elena Anguissola, Sofonisba’s younger sister who had been a student of the painter Bernardino Campi along with Sofonisba, and later entered the monastery of San Vincenzo in Mantua assuming the name of Sister Minerva.

Sofonisba Anguissola, 1551, Portrait of a Nun

From the Web Gallery of Art:

In the painting the nun wears a lily-white habit which stands out against the dark background. The red and gold bound book of prayers offers the only source of detail. The sitter is usually identified as Elena Anguissola, Sofonisba’s younger sister who had been a student of the painter Bernardino Campi along with Sofonisba, and later entered the monastery of San Vincenzo in Mantua assuming the name of Sister Minerva.