1. The gift deed is required to be stamped and registered.
2. If the gift deed made by your father is not stamped and registered then there is no gift made in the eyes of law. As such, your father continues to be the owner of the property.
3. Share transfer effected by the society on the basis of a plain paper is illegal.
4. Your father, as the owner of the land, can file a lawsuit to claim his property and also seek a stay order against your cousin to restrain him from selling the property and creating third party rights.
5. Society can also be dragged to court for transferring the flat to your cousin when he did not have the ownership thereof.