My paternal grandfather, of blessed memory, passed away last Shabbat. Both of my parents are living. My father (my grandfather’s son) is completely non-religious and does not care one way or the other regarding Kaddish. I am not close with him but know that he would readily authorize me to say Kaddish for my grandfather. My mother, a semi- ba’alat t’shuvah, has a very volatile personality. I am not close with her, but since her divorce from my father she has had very hostile feelings to his side of the family. I know that she would NOT permit me to say kaddish for my grandfather. Moreover, her reason for not allowing it--beside the family hostility issues--would be because of some perceived notion that my saying Kaddish while she is still living would "hasten" the end of her life (G-d forbid) or be an "omen" of bad things to come. What can I do?
Since your father does not care about the kaddish, his honor is not at stake. Your desire to say kaddish for your grandfather, and the honor you should give him, can be fulfilled by hiring or asking another person to say kaddish. You are not obligated to ask your mother's permission, and in fact are not obligated to listen to her if she says she does not want you saying kaddish, but in any case she would probably find out and your already stained relationship with her would worsen. For these reasons I think you should try to get someone else to regularly say kaddish. Then, if on occasion you wish to say a kaddish you may do so. When one parent dies, the son says kaddish notwithstanding the fact that the other parent is alive. This belies the idea that saying kaddish will hasten a parent's death. We normally don't take on saying kaddish as a sign of respect to our parents and an expression of our wishes for their long life, but not as a charm.