can settlement deed be cancelled

My grandfather had 3children, 2sons and 1daughter . 1son and daughter passed away 35yrs ago. My dad is the only person who alive. My dad's brother legally married a lady and they had one daughter after my uncle passed away they cut short all the relationship and they don't anything from our side . my aunt passed away had one son. My uncle had illegal affair of which he had one son they did not get married at all. my dad moved out Coimbatore 30yrs back on business purpose. so my grand parents used to stay alone and son born for my uncle they took care. Now grand fathers ancestors left 2acres of property and my grand father purchased additional 1acre of land. Without our knowledge or my dad's knowledge 8yrs ago grandfather has registered 1acre ancestors property to my uncle's illegal son saying he paid us 1lakh and we are selling it to him. After lot fight they wrote a settlement deed to me for 1acre ancesrtoral property saying he is my grandson and we are transferring the property with our full conscience. 1acre bought by grandfather they gave it to my aunt's son in settlement deed. question here : 1.Can they cancel the settlement deed written for me which registered 2.can my uncle's illegal son file a case asking for share from property as they registered ancestors property in his name as if bought it from grandfather by paying 8yrs ago 3.Does my dad any authority at on all of this ancestors property 4.can my aunt's son file suite asking for share from ancestors property We did not know the transaction for 8yrs coz we were living in different city we got to know recently when we were verifying the document.