Does depriving the nephew of disposing of an apartment he inherited from his father constitute deprivation of inheritance, and is the uncle sinful for that?

If the apartment is solely owned by your father and you inherited it from him, then your uncle or anyone else has no right to prevent you from disposing of it by renting it out or otherwise. His action, in that case, is a forbidden injustice, and you are not obligated to remain silent about it. If you have proof of your right, you can refer the matter to the competent authorities to remove the injustice from you. It is preferable in such matters involving relatives to handle them with wisdom and reconciliation rather than litigation, unless there is a fear of discord or animosity. The issue can also be presented to a respected relative who has influence.