A north Brisbane couple have been sentenced to jail this afternoon, after a court heard about how they made a pregnancy pact to inseminate their 15-year-old daughter after bearing two biological children with congenital diseases.

The 35-year-old stepfather received an eight-year prison term. He already has served 164 days in pre-sentence custody.

The girl's 39-year-old mother was sentenced to six months' jail and three months' probation.