The man, who can't be named in order to protect the identity of the girl, was found guilty on 10 charges including sexual assault and living off the avails of prostitution.
In coming to his decision, Judge Greg Koturbash weighed several days of conflicting testimony that pitted family members against one another.
The stepdaughter, who is now 18, told the court the abuse started when she was 12 years old.
She said the accused had a drug habit, and forced her into prostitution when she was 17 in order to pay for it.
Some family members testified they saw the accused naked in the same bed with the girl.
The Crown argued that the accused pimped the girl several times in Surrey and Penticton before being arrested last year.
Defence lawyers argued that the girl went into prostitution herself, and told the judge that the statements she made in court differed from statements she made to police last year.
The accused told the court that he couldn't stop the teen from meeting men for sex.
The girl's mother testified that she never saw her husband abusing or pimping out her daughter.
The judge said he found the girl's testimony to be credible, and that the stepfather's testimony was not.
The judge also said that he found the mother's testimony was not credible.
The judge ordered a psychiatric assessment of the stepfather as part of pre-sentence report.
A sentencing date will be fixed in September.