Is it obligatory to obey a mother who seeks to visit shrines, knowing that this is not permissible?

If the mother's purpose in visiting graves is to remember the Hereafter and to adhere to the Sharia rulings for visiting, then there is no harm, and you must obey her. However, if the purpose is to supplicate the dead or seek their aid, then it is not permissible to take her, and you must advise her and explain that this is major shirk, and that a servant must strive for pure monotheism.