Citigroup Inc. hired law firm Paul Weiss to investigate complaints about the behavior of the bank's wealth-management chief, Andy Sieg, who was brought on by Chief Executive Officer Jane Fraser as a high-profile hire to drive a key part of her turnaround plan. Sieg, on a quest to shake up Citigroup's wealth business, has drawn complaints from current and former staff accusing him of intimidating and unfairly sidelining employees since he arrived almost two years ago, according to people with direct knowledge of the matter. Sieg, 58, not only runs the global division, he sits on the bank's 18-person executive management team and is seen on Wall Street as a candidate to someday succeed Fraser.