What must I do to denounce the polytheistic practices at the shrine of my great-grandfather, and am I sinful if I do not intervene by going there, or is my duty limited to advising my family and those close to me?

A Muslim must do his utmost to change evil (munkar), and this varies according to his capabilities and knowledge. If the shrine is built upon your grandfather's grave, and demolishing it is possible without causing a greater evil, then you should strive for that wisely. If your relatives object, ask scholars about the ruling on evils, providing evidence. We advise bringing a qualified shaykh to the location to preach to the people and explain the danger of shirk (polytheism). Do not delay the matter until you finish your studies at Al-Azhar. You can contact the pioneers of the place individually to convince them, and strive to establish lessons on Tawhid (monotheism) in the area to strengthen people's connection with their Lord and clarify the dangers of shirk and the obligation to avoid it.