In the latest instalment, police said Monday they were looking into whether the sisters' mother Lee Myung-hee had herself abused employees verbally and physically.
Cho Hyun-min, the younger daughter who is marketing executive at the South Korean flag carrier, is under police investigation for assault after she was accused of throwing water into a man's face at a business meeting.
Cho Hyun-min has denied she threw the cup at the employee's face, saying she pushed it toward the ground and neither of the victims have reportedly expressed the wish to seek a case against the heiress.
But the family's trouble did not end there.
The customs office on Monday carried out a search of Korean Air Lines's headquarters over suspicions that the family members of Chairman Cho Yang-ho sneaked luxury goods into South Korea without paying duties.
Korean Air is not the only airline to be pulled into the spotlight in recent times.
The police and customs investigators raided the offices and homes of the Cho family this month to collect evidence.
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The boss of Korean Air has apologised for the "immature" behaviour of his two daughters and announced their resignation from the official positions they held at the company following separate controversial incidents.
The move did little to assuage public fury, with almost 90,000 people signing an online petition urging the government to ban the firm from using "Korean" in its name.
In his letter of apology on Sunday (April 22), Chairman Cho pledged to strengthen the role of the boardroom and launch a compliance committee to "institutionally prevent" such incidents from happening, while firing his two daughters.
Four years ago her older sister Cho Hyun-ah made global headlines for angrily kicking a cabin crew member off a plane after being served macadamia nuts in a bag rather than a bowl - an incident quickly dubbed "nut rage".
Cho Hyun-ah, recently named president of the airline's hotel business and her younger sister, Cho Hyun-min, a Korean Air senior executive, will be stripped of all their responsibilities, according to Yonhap News Agency. He also promised to increase the power of the company's board of directors.
He said they would be immediately removed from management. His son, Cho Won-tae, remained as president of Korean Air.