ownership of the land

gud day po! ask ko lang po, may nagsangla ng lupa sa father ko noong 1989 covered ng Emancipation Patent and in 1991, the registered owner and his wife waived and relinquished all their rights and interest over the 2 hectares of agricultural land in favor of my father my document po na pinirmahan sa Barangay, and then nanghingi na lang po ng kabayaran para sa lupa ung registered owner and the Deed of Sale was executed in 2001 and was notarized only in 2007. After the death of the vendor/registered owner in 2009, naghahabol po ung mga anak..and they are claiming that they have right over the said land. Mayroon pa po ba silang karapatan sa lupa na ibinenta ng kanilang magulang? sino po ang may karapatan at nagmamay - ari? ung father ko na nakabili ng lupa at kasalukuyang nakaposisyon at nagsasaka o ung heirs ng vendor? they used force, threat, intimidation, strategy and similar acts po on May 2, 2010 kaya inayos po namin ang pagpapatranfer ng lupa sa pangalan ng father ko at lumabas ang title only on May 11, 2010. Ano po ba ang dapat naming gawin para hindi nila gambalain ang pagsasaka ng father ko?
we need ur advice po at kung my related article po o law regarding this,pakisend nMan po para alam nMin dapat gawin..thanks! God bless!

your father should show them the title as proof that he is the owner of the property.

he can file cases of threats or unjust vexation if they continue to harass him.