It is true that not all forms of CAH are known and at the moment. What seems to be happening is that the more closely the gene sequences are studied and enzyme activity investigated has meant more forms of CAH are actually being discovered.  It is getting more complex and the biochemical effects of each type are also different.  As the knowledge base increases then so should the management of the different types.  At the moment everyone generally starts in the same place and endos do the best they can to individualise the treatment regime but it seems that in the future specific strains of CAH will be managed in different  ways because the understanding of enzyme activity is increasing. 

Your inlaws can be tested but it would probably be academic.  If none of their kids had CAH then it is more than likely that only one of them was a carrier.  My wife's mother and sisters(2) are carriers of CAHypoplasia and all had 50% chance of their sons having CAHypoplasia. Not one of the six sons got CAHypoplasia.  So they could both be carriers and have been  lucky like my in-laws..

If you are interested in a detailed analysis of where CAH stands today within the medical research community here are two papers:

Clinical Pediatrics, Jan 2001.

Congenital Adrenal Hypoplasia: From Genetics and Biochemistry to Clinical practice, Part1, Paulo Ferrez Collet-Solberg,MD. Pediatric Endocrinology, Duke Medical Center.

The same title Part 2 by the same author in Clinical Pediatrics, March 2001.

ShaunP.