Though you don’t need descendants’ permission for biographies of private people, descendants can conceivably sue for an aspect of defamation/ invasion of privacy. They may not have standing or a strong case in this legal gray area, but you’d have to deal with it. Publishing contracts are usually worded to put that on you, although in-house attorneys weigh in. If your portrayal isn’t complimentary, smart money says have a publishing attorney assess your specifics to prevent greater future expense. I recently urged a client to do that because her subject is current generation, the circumstances emotional. The longer your subject has been deceased, the safer you likely are. Is there a moral imperative to seek permission? Get their blessing? I and experts I spoke with don’t think so. What if they decline? Will you trash your project? Do consider that descendants can be great resources, confirming/correcting info and providing insights, photos, documents. You could reach out for interviews or info without asking permission. Share your angle and aim to be thorough and fair.