Opponents of the process call it "chain migration", but Melania Trump may have used the policy to sponsor her parents.
Michael Wildes, the couple's immigration lawyer, confirmed they took the oath of citizenship on Thursday.
It is believed that Melania Trump, who is a U.S. citizen, has sponsored her parents for permanent residency. "I've been a very staunch advocate against the president's policies myself as an immigration lawyer", Wildes said.
The first lady's office declined to comment.
She settled in NY in 1996 and met Mr Trump two years later.
The idea that family-based immigration could be labeled as "dirty" might have a lot to do with the Knavs' son-in-law. "This is an example of it going right", he said.
The Knavs are frequently with the Trump's and also travel with them. "Once they had the green card, they then applied for citizenship when they were eligible", he said.
The plan, which Trump reiterated last week at a Wilkes-Barre, Pennsylvania rally, would limit immigrants like his wife to sponsoring only their spouses and underage children to join them in the United States - not their parents, adult children or siblings.
The president proposed replacing most family-based immigration with a skills-based system after an attempted bombing by a Bangladeshi immigrant in NY last December. The Knavses came and went from the ceremony at a Manhattan federal building flanked by Homeland Security police.
She settled in NY in 1996 and met Mr. Trump two years later.
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.
The First Lady became a USA citizen in 2006, after she entered the United States in 2001 on a coveted Einstein visa for people with "extraordinary ability" when she was working as a model.
The Knavses' citizenship ceremony was kept hush-hush at the Jacob K. Javits Federal Building. She said the first lady's parents "are not part of the administration and deserve privacy".
The Knavses, both in their 70s, raised Melania Trump in Sevnica, a Slovenian town of around 4,500 people. Viktor was a auto dealer.