How can one deal with a relative who has been stealing since childhood, without provoking her anger or making her feel like a bad person, especially considering that her mother refuses to discuss the matter?

The girl who steals should be advised, for "religion is advice." And according to the Prophet, peace and blessings be upon him: "Whoever among you sees an evil, let him change it with his hand; if he cannot, then with his tongue; if he cannot, then with his heart—and that is the weakest of faith." The advice should be given with gentleness and kindness, clarifying the ruling on theft and the necessity of repenting from it, and opening the door of hope for her. It is also beneficial to utilize books and materials that address this crime and to supplicate frequently.