This puzzles me.
When I sold my Sherlock Holmes novel to Dodd, Mead about 30 years ago or so, lawyers for Conan Doyle's daughter (I think it was) contacted Dodd, Mead and threatened to sue if the book were ever published; they claimed control over the characters. The then-head of Dodd, Mead, who I think was a member of the Dodd family (I always think of him as Junior Dodd), told them to go ahead and sue. They backed down.
If they didn't think they were on solid ground 30 years ago, how can the estate think it has grounds now?
As an aside, I found their objections to my novel particularly laughable. They asserted that Conan Doyle would never have approved of my combining Sherlock Holmes with space travel and time travel. Given that Conan Doyle wrote a fair amount of what we now call science fiction, I wonder if his descendants had actually read much of his works.