Land used by another party without payment or sale

Hi! I need advice about my family's property.
My father died in 2007. He left 3 legitimate children from his legal wife, and three illegitimate ones including me with my mother, outside of marriage. He lived with us for the rest of his life.

One of his properties was a parcel of land. Going through his certificate of titles and documents, we discovered that a portion of that parcel of land was used by NAPOCOR for many years without due payment. My mother has been paying for the taxes of the said land, including that portion, up to now. My mother met with the NAPOCOR officials, they were amenable to using the portion of that land but they refused to pay us for that portion because they need all 6 of us children to sign a special power of attorney, authorizing one of us to transact with them. Also we could no longer retrieve the land because of the irreversible improvements done by NAPOCOR to that portion of land.

The problem is I have tried contacting my other siblings from the other family to secure an SPA, since they are in another region, but they have given me no feedback.

Is there another way to go around this, and transact with NAPOCOR without including them, or at least collect our part?

How can we settle this with NAPOCOR, when it is already evident that they have taken a portion of my father's land without payment, but still they refused to transact with us?

Sincerely seeking for your advice.

If you can prove that the same parcel of land is your share of inheritance, there is no need to execute SPA. You have to show proof that the land was yours though. Otherwise, it is presumed to be co-owned by the heirs.

I see. The title is still under my father's name, and has not been distributed to the heirs as of yet. Would that property be considered co-owned and needs an SPA signed by all of us then?

I appreciate your feedback.

Is there another legal way to get our part, at least, if my other siblings are not willing to cooperate?