My brother-in-law has corona and won’t be able to come to shul for his father’s yahrtzeit. He asked me to say kaddish for his father. I told him that it’s probably preferable that his son (the grandson of the niftar) say kaddish. Or one of my sons. Is this correct?
What you are writing is true, however the minhag is that a child doesn’t say kaddish while both parents are still alive. They can learn mishnayos, and daven for the amud. Regarding the kaddish, although you are not direct offspring, you are still a son-in-law, and you can say the kaddish.