Is the father sinful for encroaching upon a portion of his deceased brother's land, despite the sons' attempts to restore the right to its rightful owners? Does this affect the acceptance of worship? Is the son sinful if he shares residence with his father on this land? What is the correct action to take regarding this problem? And is the silence of the rightful owner considered forgiveness?

It is not permissible to transgress upon the right of another. A severe warning has been issued against whoever seizes even a handspan of land unjustly. Your father must repent to Allah and return the right to its owners. You must continue to advise him. It is not permissible to remain on usurped land, and prayer on it is a matter of dispute among scholars. You are not permitted to obey your father in dwelling on the usurped part of the land, because there is no obedience to a created being in disobedience to the Creator. As for the part of the land owned by your father, there is no harm in benefiting from it.