The Birkenstock brothers are worth $3.4 billion, even after the shoemaker's shares tanked at its IPO

• Two heirs of Birkenstock are now worth $3.4 billion after the company's IPO, Bloomberg reported.
• The German sandal brand, long mocked as producing "ugly" shoes, has seen sales boom in recent years.
• Birkenstock listed in New York on Wednesday, but shares opened at 11% below their offer price.

Two of the heirs of German sandal maker Birkenstock are now worth a combined $3.4 billion after the company went public, Bloomberg reported.