Just because it is grandfather's property, dont assume that the grand children have any right in the property.
Your grand father's house property in the instant case is his personal property. After his death, in case he has not left any WILL, all his legal heirs (i.e. sons and daughters) are entitled to get equal share in the property. If the sons and daughters (i.e. your father and his sister) have decided to give up their share to their brother (your uncle), then it is their legal right. NOBODY CAN OBJECT TO IT. NOBODY ELSE HAVE ANY RIGHT TO GET ANY SHARE IN THE PROPERTY. THEREFORE, THERE IS NO NEED WHATSOEVER TO GET ANY CONSENT/PERMISSION/APPROVAL etc. from anyone.
Therefore, the lawyers whom you approached have advised you correctly.