Complex sentence: Although Mercedes was patrilineally a French princess, she was officially also a Spanish infanta, being the niece of the Spanish Queen Isabella II, her father Antoine having been created Infante Antonio of Spain at his marriage.
Split into simpler sentences: Although Mercedes was patrilineally a French princess, she was officially also a Spanish infanta. In addition to being a niece of Isabella II, Mercedes was entitled to the title of infanta because her father Antoine had been created Infante Antonio of Spain at his marriage.