Her Wikipedia article is here. She married Friedrich, Hereditary Prince of Solms-Baruth on 1 August 1914 - World War I was just getting going at that time.
From Miss Mertens' photostream on flickr; detail-enhanced by gogm.
This dress pre-dates her 1914 marriage date based on the flaring skirt and blouson bodice. Besides, she doesn't look 25.
Keywords: Princess, German, Schleswig Holstein family, Solms Baruth family, bouffant coiffure, hat, high enclosing neckline, three quarter length full sleeves, cuffs, blouson bodice, belt, natural waistline, close skirt, pin