'Almost all Filipinos had Spanish or Spanish-sounding surnames imposed on them for taxation purposes, but a number of them have indigenous Filipino surnames. On November 21, 1849, Governor General Narciso Clavería y Zaldúa issued a decree stating that Filipinos should adopt Spanish surnames to make census counting easier.' (https://en.wikipedia.org/wiki/Filipino_name)
NOW is the time for your wife to start digging at the Registrar of Deeds across from the provincial capital for a certified copy. Since Siquijor is a separate province, might be there instead of Dumaguete. Sometimes a real estate lawyer can be helpful.
Just raised that with my wife: Apparently her sister went to Siquijor with the father and "some other relatives" to research it. I tried to sort out the detail of what my wife told me and what the outcome was, but it was tortuous and could have been in Swahili from what I could work out! I will sit down soon with her for a few hours (or days) to see if I can untangle it. But a bunch of interested relatives having already been there indicates to me it is either untraceable, they now own it or it is worthless. My hope that a series of 28-floor condos were built on it and the owners now owe the family millions, is surely in vain - especially as it is in Maria!
Good fn luck. Just need to "bite the bullet" and work on it until you get it, unlike a lot of Filipinos... We were once interested in a lot with 7 untraceable owners (unknown number of whom were deceased)... Walked away, found one with ONE live interested seller. Only needed one notarized letter from farmer there that he did not live there. One of our friends paid for a lot four years ago, been asking the seller for a deed for four years, sad...