Hey MTF, sorry to hear about the troubles. It's hard when it's someone you love and you can't do much.
The thing I would still be concerned about, is what caused the shock, head pain, and disorientation originally. She wasn't on the prednisone at that point. In regards to the aneurysm, be it carotid, thoracic, or abdominal, it's a weakening of the vessel wall, that's all. The weakness in wall wouldn't give her those symptoms. This may sound weird right now, but she actually "lucked out" by having them find the carotid aneurysm. Most people that have one don't know it until it bursts, which has a high fatality rate. This may have saved her life in the long run! I would still be concerned about the original issue unless it was actually brought on by stress/anxiety.
I have a friend who's dad was being scanned for something and inadvertently they found a thoracic aortic aneurysm. He was at Stanford. They told him it can be genetic. He has two sons, my friend and his brother. Both were tested and my friend has a small thoracic aortic aneurysm. His brother does not. The doctor mentioned that some people are born with it. I'm telling you this since it can be genetic and you mentioned children. I'm not sure at what age they start testing, but it's something you'll want to ask the doctor.
I wish you guys well and hope everything keeps trending in the right direction and the surgery goes great!!