Is it permissible to help one's father complete a septic tank for the house, which is partially located on the neighbor's land, knowing that the father is not convinced of the impermissibility of that and becomes angry if not helped?

Your question is unclear, but if your father is digging a hole on his land that encroaches upon his brother's land without his permission, then he has no right to do so. He is obligated to repent to Allah and return the right to his brother, because encroaching upon another's land without right carries a severe warning. The Prophet (peace and blessings be upon him) said: "Whoever usurps even a hand span of land unjustly, Allah will encircle him with it from seven earths on the Day of Resurrection." It is not permissible for you to assist your father in this aggression. However, helping him in his permissible work and digging his own land is permissible, and you must continue to honor him and treat him kindly.