By listing all Cristy, Cristina, Christiane working at Delgado, it should be easy to find a Caballero (last name or middle name) by method of elimination. Not unless she has left for the US, the sister should know where the brother is. Even the colleagues of the sister in the hospital would know the whereabouts of the sister if ever she may have left the hospital to work somewhere else.
Being an engineer, he should have a file at the PRC where all engineers are listed or if he is employed, he should have a file at SSS or GSIS. If he went abroad, there is a file at the POEA.
You can also list all the schools offering engineering courses and surely, there is a record of an Antonio Caballero who got his engineering degree there.