Am I obliged to give money to my well-off father, who spends it on things harmful to him (such as cigarettes and qat), while my other relatives are more in need of it?

You are not obligated to give your father from your money if he is self-sufficient. However, it is preferable to give him to gladden his heart. Nevertheless, if you know that he spends money on prohibited things such as cigarettes and qat, do not give him, but rather spend the money on your needy relatives, while gently apologizing to him and continuing to advise him to quit those forbidden habits.