Twenty years ago, a glamorous platinum-blond widow arrived at the Paris law office of Claude Dumont Beghi in tears. Someone was trying to take her horses — her “babies” — away, and she needed a lawyer to stop them. She explained that her late husband had been a breeder of champion thoroughbreds. The couple was a familiar sight at the racetracks in Chantilly and Paris: Daniel Wildenstein, gray-suited with a cane in the stands, and Sylvia Roth Wildenstein, a former model with a cigarette dangling from her lips. They first met in 1964, while she was walking couture shows in Paris and he was languishing in a marriage of convenience to a woman from another wealthy Jewish family of art collectors. Daniel, 16 years Sylvia’s senior, already had two grown sons when they met, and he didn’t want more children. So over the next 40 years they spent together, Sylvia cared for the horses as if they were the children she never had. When Daniel died of cancer in 2001, he left her a small stable. ...