What should a son do who spent his money on building a house for his parents, then his parents expel him from his own apartment in the same house to give it to his younger brother?

A father may take from his son's wealth when he is in need or if the son is wealthy, provided two conditions are met: that he does not cause hardship to the son or take what the son needs, and that he does not give what he took to another child. Therefore, your father does not have the right to force you to relinquish your property to your brother, and your refusal is not disobedience. However, be gentle in your refusal and remind them of Allah.