He was married to Anastacia Espinosa, daughter of Francisco Antonio Espinosa and Francisca Ramírez. His marriage ceremony was held in the Chapel of Morón by the priest Juan Antonio Merlo. He and his wife were parents of Agustina, Juana Ventura, Juan Napomuceno, Ramón Florencio, Cipriano, Francisco Javier, Cornelio, Mariano and Saturnino Alegre, who were powerful landowners of the centre-east of Buenos Aires. Almost all the ancestors of Casimiro Alegre served in the provincial militias. His grandfather Bernabé de Alegre, was an officer who reached the rank of sergeant major of the Spanish Army. His illustrious ancestors include, among others, Esteban Alegre, who attended the foundation of Buenos Aires by Juan de Garay, and Luis Alegre, a conqueror and expeditionary of Flemish origin, who arrived in the expedition of Pedro de Mendoza. He was also a descendant of Francisco de Paredes, a Spanish conquistador native of Burgos, who arrived at Río de la Plata on the first expedition of Mendoza. His wife Anastacia Espinosa, was a direct descendant of prominent Spanish officials, including Antón Higueras de Santana, Toribio de Peñalva and Juan de Garay y Becerra. His son Cipriano Alegre Espinosa, was married to María Fermina Martínez, daughter of Joseph Martínez Muñoz and Catalina Roberto Jaimes, a Creole woman, granddaughter of Joseph Roberto, a soldier originally from Milan, who served in the Buenos Aires Fort towards the end of the 17th century. The Martínez Roberto family was related to Bartolomé Jaimes, an Andalusian conqueror born in Huelva, and of Francisco Reynolds, belonging to an English-Creole family, linked in turn with the Valencian soldiers, who served in the Spanish Tercios at the service of Philip V of Spain. Other soldiers linked to this family were Gerónimo Vieyra, a Portuguese gunner who served in the Fort of Buenos Aires towards the beginning of the 18th century, and Ramón Sorayre, a who participated in the Argentine Civil War. Casimiro Alegre was a great grandfather of Juan Bourré Andrade, a lieutenant colonel of the Argentine Army who participated in the Conquest of the Desert.