Property gifted by owner can be taken back by him

Respected Sir, My in laws are having a property in my village and was gifted by my aunt's brother. It was actually a land sold to my uncle. To avoid the tax he gifted the land to my aunt and registered the same on my aunt's name in the year 2004. My uncle has constructed the house and paying the taxes to the government. Recently the one who sold the land(my aunt's brother) is trying to encroach the house and he stated that it was gifted, but still he has the right to take back the property. That was actually gifted land, but now he is claiming the whole house. My question is 1. How legally he can claim the property ? 2. What is the validity of the gift deed? 3. To avoid further problems with him what I can do at this position.? Can they do the gift deed to my wife ? or registration of house on my wife's name ?