You said you found them on the 1880 census, but you didn't give what you found.Since you did give the name of the county I went and looked. With names and ages I found the following. Modesto's DC gives his mother's maiden name.
Name: Modesto De La Garza Death Date: 17 Jun 1933 Death Place: Karnes Co, Texas Gender: Male Race: Mex Death Age: 64 years Estimated Birth Date: 1869 Birth Date: Birthplace: Mex Marital Status: Widowed Spouse's Name: Father's Name: Domingo De La Garza Father's Birthplace: Mex Mother's Name: Viviana Toejo Mother's Birthplace: Mex Occupation: Merchant Place of Residence: Cemetery: Burial Place: Kenedy, Tex Burial Date: 18 Jun 1933 Additional Relatives: Film Number: 2115548 Digital Film Number: 4163871 Image Number: 2426 Reference Number: cn 28732 Collection: null
This looks like another child, born after the 1880 census, but with name of mother as her married name.
Name: Laura G. De Rustamante Death Date: 14 Jan 1938 Death Place: Kenedy, Karnes, Texas Gender: Female Race: Death Age: 52 years Estimated Birth Date: 1886 Birth Date: Birthplace: Marital Status: Married Spouse's Name: Gregoria Rustamante Father's Name: Domingo Dela Garza Father's Birthplace: Mother's Name: Vivian T. Del La Garza Mother's Birthplace: Occupation: Place of Residence: Cemetery: Burial Place: Burial Date: Additional Relatives: Film Number: 2117655 Digital Film Number: 4030401 Image Number: 996 Reference Number: cn3457 Collection: Texas Deaths, 1890-1976
By the 1900 census Duvual Co, TX, Domingo is widowed and there are 3 more children listed, including the above Laura and your wife's Juan.