From Jon Jermey, moderator of the GADetection Forum:
Unfortunately, from a legal perspective, it doesn’t matter if the copyright holder is sick, dead or indifferent, or how many steps of inheritance removed they are from the original author. Even if they don’t KNOW they are the copyright holder, they can still find out twenty years later and bring suit for any monies made by the hard-working people who have actually bothered to make the books available to the public. If [an author’s] daughter’s cousin’s son’s spouse’s nephew—or whatever—doesn’t want the books published in the US then what they say goes. And of course if there are joint heirs over several generations then the whole thing gets even more insanely fraught.
I was always taught as a child that rights had to be earned. Copyright seems to be the exception to the rule.