SPOILERS for Win by Harlan Coben

 

If you continue reading, you will be exposed to spoilers about the above book. Proceed with caution.

It was Win's cousin, Patricia, who killed her father. Her father was a member of the Jane Street Six. He went off to Brazil until the smoke cleared. But there he met Patricia's mother. 

Patricia had a thing going with Ry and they stole the artwork. While they had the artwork hidden, she found proof hidden in the artwork that her father was involved in the Hut of Horrors. And that is why she killed him.