A lawyer for Viktor and Amalija Knavs says the Slovenian couple took the citizenship oath on Thursday in New York City.
"This golden experiment, these doors that are in America, remain hinged open to attractive people as they have today", Wildes said.
Viktor Knavs is 74, two years older than his son-in-law.
Asked if the Knavses had obtained citizenship through "chain migration", their lawyer said, "I suppose. This is an example of it going right", he said. Wildes told the newspaper that the Knavs fulfilled the five-year requirement, but declined to give further comment. They tend to keep a low profile despite spending some time at the White House, much like how former first lady Michelle Obama's mother lived at the White House where she focused on helping raise the Obamas' daughters.
The president often rails against family-based immigration at his rallies, and has called it a pathway for terrorists to enter the country. He also called for eliminating a visa lottery program for people from countries under-represented in the U.S.
The US allows a number of ways for US citizens and legal permanent residents to sponsor family members to come to the US permanently, including categories for parents, adult siblings and adult children, married and unmarried.
Since initial reports emerged in February that the Knavses had obtained permanent residency in the United States, there has been a lack of clarity about when or how the couple received green cards. Thomas Cioppa, New York district director of the US Citizenship and Immigration Services, presided over the 20-minute ceremony, Mr Wildes said.
Canada has a MERIT-BASED system, while the United States prioritizes more FAMILY-based immigration.
Typically, naturalisation ceremonies at the Jacob J Javits Federal Building at 26 Federal Plaza are large events, where groups of immigrants are sworn in as citizens en masse, after reciting an oath and the Pledge of Allegiance.
Trump hasn't commented or tweeted about his in-laws' newfound citizenship.
She settled in NY in 1996 and met Mr. Trump two years later.
Amalija is 73. The first lady, born Melanija, changed her name to Melania Knauss when she started modeling.
Around the same time the Knavses were completing the ceremony, he was tweeting his rage about the ongoing Russian Federation probe ("an illegally brought Rigged Witch Hunt") and his excitement about plans for a sixth branch of the military ("Space Force all the way!").