1. Since the properties of your grandfather has not been physically partitioned between your father and uncle , those are considered to be joint property wherein all of you have equal shares.
2. Even if you or your cousins have share in it , due to non partitioning each of you has right of possession in each inches of the property.
3. In that situation you have legal right to stop your cousins to make any further constructions in the joint properties and claim your share in the property as well.
4. To do that you have to file a suit for partition and for injunction.
5. Meet a local lawyer asap.