A special inspector charged with looking into allegations of corruption involving presidential aides and close family members has sought a formal investigation into President Park Geun-hye's estranged younger sister over fraud, state prosecutors said Tuesday.

Lee Seok-su, the inspector under the presidential office, filed the report on Park Geun-ryoung with the country's top prosecutor Kim Soo-nam last month. The case has been allocated to the Seoul Central District Prosecutors' Office, according to the prosecution.

The younger Park is facing the investigation for allegedly exaggerating her influence to receive large sums of money, which she reportedly failed to return. She was also fined 5 million won ($4,400) late last year on different fraud charges.

The former head of the Korean Children’s Center has had a falling out with her older sibling over various issues in the past, including her marriage to a controversial figure. Relations between the sisters are very strained.

Last week, Lee also filed a case against Woo Byung-woo, senior presidential secretary for civil affairs, over a string of corruption allegations raised against him.

Lee himself has been mired in controversy after local broadcaster MBC reported last week that he illegally leaked information about his investigation into Woo to a local reporter via social media.

By law, a special inspector can face a prison sentence of up to five years for leaking information related to an ongoing case. (Yonhap)