Charles Laub, 44, who was convicted by a Cedar County jury Friday, faces seven years in prison when he is sentenced March 11.
The sisters, Sara and Heather Staples, 27, said they were strictly confined after undergoing a religious marriage ceremony in Utah. They lived in Cedar County, but were not legally married.
The women said they were not allowed to decline sex and if they did it was met with force, though prosecutors dropped a rape charge when one sister said under oath she couldn't recall if Laub ever actually raped her.
Laub fathered eight children between the two women, the Springfield (Mo.) News-Leader reported Monday.