From the lost gallery’s photostream on flickr. Infanta Isabel looks like a no nonsense Lady - a female Jean Luc Piccard.
Isabel wears a dress similar to one seen in the this image and the same jewels. Hanging sleeves could be hiding in the dark parts of the portrait.
Keywords: Rubens, Infanta Isabel Clara Eugenia, Princess, Spanish, Habsburg family, straight coiffure, jeweled headdress, high enclosing neckline, neck ruff, tabbed sleeves, elbow length full upper sleeves, long tight lower sleeves, ruffled cuffs, necklace, earrings, jeweled crucifix, pendant, fan