My husband and I are both carriers.  We have an 11 yr old daughter with swcah.  I have a son 21 yrs old who does not have cah ( could be a carrier).  I have a younger sister who has 4 children (3 boys and 1 girl) and a younger brother who has 2 children (1 and 1) and none of them have cah or related symptoms (any of them could be carriers maybe).  There are no cousins on my husbands side of the family to compare to.  No one else in the family on either side has ever been diagnosed with cah or symptoms of the condition.  Such as late onset....

Regarding your sister in law, If it were me and I was pregnant for the first time just knowing anything genetic is in the family I might want to get checked just to be sure.  My guess tho is that there is probably nothing for her to worry about. 

As a carrier myself and having been in the "cah community" for about 10 years, I honestly dont think I have ever heard of a cah child’s aunt or uncle who were both carriers then having a cah child.  Maybe others on the board have heard of that.

I am sure there are much better and more complex genetic explanations to calculate the possibilities. This is only my opinion.

Tell your sister congratulations on the news!