I had the same question when my son was diagnosed with CAH at 2 weeks of age.  I was assured by a geneticist that if my two-year old had CAH he would also have the salt-losing version and therefore, since he was healthy there was no need to test him. 

As mentioned above, I do think it is possible, but unlikely, for siblings to have different forms of CAH.  So, if it would give you peace of mind, then I think you might as well have your older daughter tested.  Also, if your daugther was screened at birth, maybe you could find out from your pediatician or state health department what the screening results were.  If the 17-OHP was in the high range of normal that might give you more reason to have her tested. 

As the others have said, CAH is a very manageable disease.  My son is now 2 and is doing really well.  But I remember the first few months having so many questions and concerns about his future.  It was a really difficult time.  This board was a Godsend to me and I'm sure you will find it helpful also.