A Brisbane teenager was quoted religious scripture and banned from taking the sacrament as punishment for molesting a female relative after the girl's parents decided against reporting the offence to police, a court has been told.

The family instead left it to a church bishop to discipline the boy, but the strategy failed and the teenager went on to commit multiple sex offences against young girls over a 15-year period, ending with the stalking of his wife's 16 year-old sister last year.

Now aged 29, the man admitted at Brisbane's District Court yesterday to spying on his sister-in-law on eight separate occasions while she used the bathroom.