Is there any documented cases of somebody taking the ideas or writings of a deceased relative as their own? That is, someone dies leaving behind a novel or collection of songs or such that they never mentioned to anyone for whatever reason, and a surviving descendant claims it as their own.
Not documented completely, and not a relative, but there is a belief that Mikhail Sholokov plagiarized his novel And Quiet Flow the Don from Fyodor Kryukov. The evidence is inconclusive.
I’ve never heard it happening for a relative. Usually, if published, the relative will be honest about who wrote it (e.g., A Confederacy of Dunces).