Still, one brother discussed new venture with major donor before Biden entered the White House. In the midst of his campaign for president, Joe Biden took his younger brother, Frank, aside to issue a warning. “For Christ’s sake, watch yourself,” Biden said of his brother’s potential business dealings, according to a person with knowledge of the conversation. “Don’t get sucked into something that would, first of all, hurt you.” Biden, whose tone was both “jocular and serious,” according to the person, seemed to know then what is becoming plainly obvious now: His family’s business ties threatened to undermine an administration...