My son was diagnosed when he was around 9 days old.  He was tested at birth, but the results did not come back before he was deathly ill.  My ped. figured it out from the symptoms.  He had failure to thrive.  Would not eat, and he just slept.  He had some mild vomitting, but I do remember the doc. saying he was suprised it wasnt worse because that is a sign.  He never gained weight only lost weight.  He lost over a lb in the first week.  My son is a salt waster too, so if the child isnt a salt waster the symptoms are probably much less severe.  I will mention that it is rare for two people that carry it to get together especially in the same family.  The odds are probably slim that your sisters son has it, but I understand your concern.  Is her doctor aware of it being in her family? 

Carrie