Renee,
R they only going to military doctors, that could explain a few things?
They only advice I could give u is to use tough love, when your SIL calls u whinning about her son, just tell her u don't want to hear about it until u gets a CF test. That sounds bad, but she sounds pitiful for not wanting to be an advocate for her child and do the right thing. Doctors are human. They do not make enough money and they are not all the most intelligent people in world. While they know way more than I do, becasue they have went to school 10 plus years and this is their job everyday, they still don't live with your child, nor know everything about them unless u tell them, which your sister may not be telling the doctors all.
Lori