Complex sentence: Though Knox admits that the girls were probably duped, someone has to go to jail for their crime, and as long as no Nick Parks (obviously an alias) can be produced to clear the girls, they will finish out the remainder of their sentences.
Split into simpler sentences: Though Knox admits that the girls were probably duped, he firmly believes that someone has to go to jail for this crime. As long as no Nick Parks (obviously an alias) can be produced to clear the girls, he says, they will finish out the remainder of their sentences.