Seven months before Melvin Simon died, the ailing shopping-mall magnate altered his $1 billion will during a three-hour meeting in Asherwood, his palatial home near Indianapolis. A financial adviser had to hold and guide his hand as he signed the revised document, according to court filings.

Now that scene has become the center of a bitter estate struggle pitting Mr. Simon's second wife, Bren Simon, whose inheritance was greatly increased by the new will, against his children from his first marriage.