There are many versions of the name Santillan, Santilla, Santillana, Santillanes, Santillanez, Santilli. The way I understand it the Santillan family regardles of the spelling all came from northern Spain. This family must be restless for there are santillanes from Spain to the Phillipines, Peru, Ecuador, Mexico, to the United States the largest concentration of the Family that I have heard of is in New Mexico along the Rio Grande River, from Las Vegas, NM to the North to Soccoro in the South.
A story I heard as a child from my grandfather was that there were seven brothers that emigrated to the New World in the late 1600's some of them stayed in what is now New Mexico. I have a second cousin that lives in Mexico City (I told you we are restless) his name is Eduardo Franco, he has been also trying to track down the history of the family.