I have the following information regarding your request:
Isabel Santillan was born Isabel Lopez in San Luis Potosi, Mexico.
Her parents were Juan Lopez and Bibiana Guzman, born 18 Dec 1883 in San Luis Potosi, Mexico, died April, 1985 in Houston, TX., (SSDI at www.rootsweb.com)
Bibiana Guzman is the daughter of Selestina Sanchez and Jose Jesus de Guzman of Armadillo de los Infantes (marriage date 31 May 1860), San Luis Potosi, Mexico (www.familysearch.org)
Juan Lopez is the son of Andres Lopez and Simona Infante (Ynfante) marriage date 21 Mar 1859, also in Armadillo de los Infantes, San Luis Potosi (www.familysearch.org). As you can see, both families had been in San Luis Potosi for quite some time.
Isabel Lopez married Felipe Santillan, son of Rafael Santillan and Delores Lopez. Rafael and his sons Felipe and Agustin came to Texas from Morelia, Michoacan, Mexico in 1915 (1920 US census, Harris County, TX).