Viktor and Amalija Knavs, both in their 70s, took the citizenship oath at a private ceremony in New York City.
"This golden experiment, these doors that are in America, remain hinged open to lovely people as they have today", Wildes said.
She settled in NY in 1996 and met Mr Trump two years later.
Asked if the Knavses had obtained citizenship through "chain migration", their lawyer said, "I suppose".
He said: "It's a dirty - a dirtier word", but added: "It stands for a bedrock of our immigration process when it comes to family reunification".
The First Lady wears an nearly "naked" dress while accompanying her husband to the North Atlantic Treaty Organisation summit.
Viktor Knavs, a auto salesman in Slovenia, and Amalija, who worked in a textile factory, are over 70 years old, retired and pass much of the year in the United States, where they regularly spend time with their daughter and grandson Barron.
Melania Trump became a citizen in 2006 after obtaining a green card.
Her spokeswoman, Stephanie Grisham, declined to comment.
The idea that family-based immigration could be labeled as "dirty" might have a lot to do with the Knavs' son-in-law. While the president never names Saipov, who obtained his green card through the equally maligned diversity lottery, which grants visas to people from countries that have had fewer immigrants, he has been known to detail the attack.
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.
Wildes wouldn't say how long the Knavses have lived in the US.
The Knavses' citizenship ceremony was kept hush-hush around the Jacob K Javits Federal Building.
She settled in NY in 1996 and met Mr. Trump two years later.
He maintains there should instead be a merit-based system, which prioritises professionals over relatives, and has drawn censure for his strident attacks on immigration laws and immigrants.
Melania Trump's parents have been sworn in as United States citizens.
Viktor Knavs was a auto salesman in the Slovenian town of Sevnica, while his wife Amalija worked at a textile factory.
Melania Trump was born in 1970 and during her childhood Slovenia, then part of Yugoslavia, was ruled by Josip Broz Tito, a communist dictator who nonetheless allowed more freedoms than other Eastern bloc leaders.