I suggest his daughters are worried about their share of inheritance, and somebody convinced them to make this claim.
Probably lawyers, many of whom would gladly take advantage of fairly unsophisticated people.
B.B. King himself probably gained a good degree of common sense through his long life, but I doubt that some of his many offspring would do so.
Common sense means 89 is the end, period. I read or heard that in recent years, his live performances basically consisted of him "opening" for others, by playing a few bars at the start, and then sitting down and listening only, for the remainder of their performances. I.E. he could no longer perform the full sets he used to do.
He is lucky to have lived such a long and productive life. RIP.