Uncle sold his share of land to my father,children asking for share

Hi Sir/Madam My grandfather shared his property to my father and his brothers. Later when my uncle(my father's elder brother) was in necessity, he sold his share of the property to my father in the presence of my grand parents and registrar. At that time my cousins( father's elder brother's kids) were minors. Right now, the property is on my name. Now after many years (say 15 years) they are claiming for the property. Is it valid? If so, how? If not so, what should I do? Please help me for the same.