What does the portrait of Albert’s mother reveal to Monte Cristo?

The portrait reveals Dantes' former fiance, Mercedes.... Albert's mother.

This portrait attracted the Count of Monte Cristo's attention, for he made three rapid steps in the chamber, and stopped suddenly before it. It was the portrait of a young woman of five or six and twenty, with a dark complexion, and light and lustrous eyes, veiled beneath long lashes. She wore the picturesque costume of the Catalan fisherwomen, a red and black bodice, and golden pins in her hair. She was looking at the sea, and her form was outlined on the blue ocean and sky. The light was so faint in the room that Albert did not perceive the pallor that spread itself over the count's visage, or the nervous heaving of his chest and shoulders. Silence prevailed for an instant, during which Monte Cristo gazed intently on the picture.

"You have there a most charming mistress, viscount," said the count in a perfectly calm tone; "and this costume—a ball costume, doubtless—becomes her admirably."

"Ah, monsieur," returned Albert, "I would never forgive you this mistake if you had seen another picture beside this. You do not know my mother..."