Private investment firms in the U.S. that manage the wealth of a single family are winning the right to service in-laws and their relatives.

For the descendants of William E. Simon, Treasury Secretary under Presidents Richard Nixon and Gerald R. Ford in the 1970s, that means they can keep managing money for Meryl Streep and her foundation within their family office.

The Simons, who have advised their former sister-in-law for 26 years, sought permission from the U.S. Securities and Exchange Commission after a 2011 rule tightened the definition of family clients. The regulator on Jan. 20 granted their petition, according to an SEC document that didn’t identify Streep. Two people familiar with the matter said the former sister-in-law is the Oscar-winning actress, whose brother Dana was recently divorced from Mary Simon, a daughter of the ex-Treasury Secretary.

The Streep decision followed similar orders granted to the offices that serve the billionaire heirs of energy tycoon Dan L. Duncan and descendants of the New York banking dynasty of Joseph S. Gruss. By winning SEC approval, the single-family offices are redefining how broad they can be without divulging information about the wealth they manage or triggering regulatory scrutiny.

“There isn’t any real compelling reason for the public to know how a family office operates,” said David Guin, a partner at Withers Bergman who specializes in securities regulation for family offices. “If you’re a commercial adviser, the purpose of registering is to make sure there is a certain level of available information and compliance in place for their investors’ protection.”

SEC Registration

Family offices, which have existed for more than a century to handle the financial affairs of the wealthy, oversee an estimated $4 trillion globally. They typically don’t have to register with the SEC like other money managers, avoiding certain compliance costs. They also don’t have to disclose equity holdings even after rules enacted in the wake of the 2008 financial crisis increased oversight of investment advisers.

Keeping their privacy came with a catch. When the SEC defined family members in rules issued in June 2011, it said families could designate a common ancestor, whose descendants could be served by the office. The designation of that person rather than a husband and wife, means providing services to a spouse’s parents, siblings or nieces and nephews could force registration as a commercial investment adviser.

“We’ve had to restrict who’s been able to invest through the family office,” said Bradley Van Buren, an attorney and partner at Holland & Knight in Boston who works with family offices.

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