Cho took office earlier this month despite a heated debate among political parties and the general public over suspicions regarding his daughter's education and a family fund investment.
The relative, also surnamed Cho, is at the center of allegations about the family's suspicious investments in a private equity fund.
He was arrested at Incheon International Airport on Saturday on embezzlement and other charges after being out of the country for about a month.
He is considered the de facto owner of Co-Link Private Equity, the operator of a fund to which Cho's wife and two children, as well as his brother-in-law and his two children, have invested a combined 1.4 billion won ($1.2 million).
Cho, the son of one of Cho Kuk's cousins, is accused of rigging stock prices, embezzling billions of won from companies invested in by Co-Link.
He is also suspected of running the private equity firm without officially registering as its chief executive, among other charges.
The private equity firm's website introduces three company executives, but Cho is not one of them.
The official chief executive of the firm has also been questioned by prosecutors. But the court denied prosecutors' request to arrest him, saying the charges are not serious enough.
The fund invested most of the money from the Cho family in Wealth C&T, a local lamppost switch manufacturer, two months after Cho became the senior presidential secretary for civil affairs in 2017. The company's sales soared after the investment, thanks to massive orders from government offices.
Prosecutors are expected to focus on whether Cho's wife unjustly intervened in the investment process and whether there were any irregularities in the fund investment.
The justice minister has claimed that the family was not aware of the companies the fund invested in and the role the cousin played in the private equity firm.
Meanwhile, in a separate case, the prosecution is also looking into allegations that Cho's wife fabricated a university award to help their daughter enter medical school.
Cho, who has denied most claims against his family, has pledged not to intervene in the probe into his family and ordered the prosecution to conduct the probe in a fair and strict manner.
In South Korea, the justice minister oversees the prosecution in terms of human resources and administration. (Yonhap)