They were the daughters of Infanta Blanca of Spain and Archduke Leopold Salvator, born in 1894 and 1899, respectively. Archduchess Margarethe’s Wikipedia article is here and Archduchess Maria Antonia’s is here. Margarethe, an exile after the fall of the Austro-Hungarian Empire, married an Italian diplomat, Marchese Francesco Maria Taliani de Marchio, late and did not have children. Maria Antonia went into exile to Spain with her family and married Ramon Orlandis y Villalonga who died in the Spanish Civil War. She emigrated to South America. She married Argentine-born Don Luis Perez Sucre in Uruguay in 1942. Wikipedia’s article for Maria Antonia has a photo of her wearing the same dress with a dog dated 1913.
From eBay; sepia tone removed.