Crown prosecutor Marie Grills said it was offending at the highest scale, and sought a minimum 16-year jail term.
"The Crown would say the planning and premeditation was at a high level," she said.
Defence counsel David More agreed to the 16-year term, and said there were no aggravating or mitigating factors.
Judge Kevin Phillips told the man the court needed to impose a term that would denounce his offending.
"The scale of offending is high, the breach of trust is high, the nature of the violation is high," he said.
The man was sentenced to 18 years imprisonment, with a minimum non-parole period of 10 years.
Earlier, the two victims gave emotional testimonies in court.
The elder victim said she over-exercised and stopped eating during the time of the offending, and weighed just 18kg when she was 13.
The younger victim started self-harming at 10, and spent some time in a mental health facility in Christchurch.
Judge Phillips said it would be difficult to imagine a higher breach of trust.
"They were entirely and absolutely vulnerable."
