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.

Integral Part

In the case of Streep and the Simons, the SEC accepted the family’s plea, which argued that the former sister-in-law is an integral part of the family whose assets were managed by them for more than two decades, according to the filing. That familial relationship shouldn’t mean that the firm is a commercial investment adviser, according to the petition, which didn’t disclose the assets overseen by the firm.

Peter Simon, a son of William E. Simon and a co-founder of their namesake firm in Morristown, New Jersey, declined to comment. Streep’s publicist at 42West, didn’t respond to a request for comment.

The Simon’s family office was among a handful of firms who filed applications with the SEC after the rules related to in- laws changed in 2011. They are also among the first to have their requests granted, after a more than two-year process.

Duncan Family

The SEC approved in July a petition from the Duncan Family Office, which sought to keep advising a mother-in-law who had been a part of the family for at least 16 years, according to the filing. The firm serves billionaire descendants of Dan L. Duncan, who founded the Houston-based energy company Enterprise Products and died in 2010. Duncan’s four children have a combined wealth of $22.8 billion, according to the Bloomberg Billionaires Index.

That same month the regulator granted an exemption for the Gruss family. The financier founded New York-based Gruss & Co. and specialized in oil and gas industry mergers. The family office won concessions to keep advising sisters-in-laws and their families.

Martin Lybecker, an attorney at Perkins Coie in Washington, who represented the Duncan and Gruss family offices in their applications, declined to comment on the families or the SEC’s decision.

Wick Family

Families have to apply to the SEC on a case-by-case basis. There’s at least one application pending, from D-W Investments, which provides services to the descendants of Myron A. Wick, Jr. The petition asks to keep advising a sister-in-law and a trust that she’s a beneficiary of, according to the document. Wick was the former chief executive officer of Dominick & Dominick, the investment banking house, according to a 1990 obituary in the New York Times.

The 2011 rules will be an issue for more family offices over time who want to expand their clients without registering. That’s because the designation of a common ancestry will affect in-laws on one side of a familial lineage with each successive generation, Lybecker wrote in a 2013 book titled “Investment Adviser Regulation.”